Positive quotes

Variant: When I was 5 years old, my mother always told me that happiness was the key to life. When I went to school, they asked me what I wanted to be when I grew up. I wrote down ‘happy’. They told me I didn’t understand the assignment, and I told them they didn’t understand life.

As quoted in Business Etiquette for the Nineties : Your Ticket to Career Success (1992) by Lou Kennedy, p. 8
Variant: Always be a first rate version of yourself and not a second rate version of someone else.

Person to person: The problem of being human: A new trend in psychology (1967)
Source: page 187.

Variant: You must give up the life you planned in order to have the life that is waiting for you.

Variant: Above all, be the heroine of your own life, not the victim.

Source: Meditations

Variant of this quote "The best way to cheer yourself is to cheer somebody else up." is misattributed to Albert Einstein.
Source: According Quote Investigator Mark Twain did write a version of this saying in a personal notebook in 1896, and it was published by 1935 in “Mark Twain’s Notebook”. https://quoteinvestigator.com/2012/12/21/cheer-somebody/

Variant: Today is your day, your mountain is waiting. So get on your way.

Variant: Be a rainbow in somebody else's cloud.
Source: Letter to My Daughter

Source: Leaves of Grass

Letter to his son Eduard (5 February 1930), as quoted in Walter Isaacson, Einstein: His Life and Universe (2007), p. 367
1930s

This has become attributed to both Walt Whitman and Helen Keller, but has not been found in either of their published works, and variations of the quote are listed as a proverb commonly used in both the US and Canada in A Dictionary of American Proverbs (1992), edited by Wolfgang Mieder, Kelsie B. Harder and Stewart A. Kingsbury.
Misattributed

Source: Their Eyes Were Watching God (1937), Ch. 3, p. 21.

1 September 1875, page 226
John of the Mountains, 1938
Context: How infinitely superior to our physical senses are those of the mind! The spiritual eye sees not only rivers of water but of air. It sees the crystals of the rock in rapid sympathetic motion, giving enthusiastic obedience to the sun's rays, then sinking back to rest in the night. The whole world is in motion to the center. So also sounds. We hear only woodpeckers and squirrels and the rush of turbulent streams. But imagination gives us the sweet music of tiniest insect wings, enables us to hear, all round the world, the vibration of every needle, the waving of every bole and branch, the sound of stars in circulation like particles in the blood. The Sierra canyons are full of avalanche debris — we hear them boom again, for we read past sounds from present conditions. Again we hear the earthquake rock-falls. Imagination is usually regarded as a synonym for the unreal. Yet is true imagination healthful and real, no more likely to mislead than the coarser senses. Indeed, the power of imagination makes us infinite.

Variant: Success is getting what you want. Happiness is wanting what you get.

As quoted in The Gigantic Book of Teachers' Wisdom (2007) by Frank McCourt and Erin Gruwell, p. 410

Source: His Dark Materials, The Golden Compass (1995), Ch. 18 : Fog and Ice

Source: Island Beneath the Sea

This quote is often misattributed to Lincoln. The earliest instance that Quote Investigator could locate was "in an advertisement in 1947 for a book about aging by Edward J. Stieglitz, M.D". The advertisement for “The Second Forty Years” which ran in the Chicago Tribune newspaper read like this: The important thing to you is not how many years in your life, but how much life in your years! (Compare 1947 March 16, Chicago Tribune, “How Long Do You Plan to Live?”, [Advertisement for the book "The Second Forty Years" by Edward J. Stieglitz, M.D.], p. C7, Chicago, Illinois. (ProQuest)). Source of misattribution: It’s Not the Years in Your Life That Count. It’s the Life in Your Years - Abraham Lincoln? Adlai Stevenson? Edward J. Stieglitz? Anonymous? by Quote Investigator on July 14, 2012 http://quoteinvestigator.com/2012/07/14/life-years-count/
To my way of thinking it is not the years in your life but the life in your years that count in the long run.
Adlai Stevenson II, Address at Princeton University, "The Educated Citizen" (22 March 1954) http://infoshare1.princeton.edu/libraries/firestone/rbsc/mudd/online_ex/stevenson/adlai1954.html. This has also been paraphrased "What matters most is not the years in your life, but the life in your years" and misattributed to Abraham Lincoln and Mae West.
Adlai Stevenson II, "If I Were Twenty-One" in Coronet (December 1955).
Misattributed
Variant: It is not the years in your life but the life in your years that counts.

Hawthorne and His Mosses (1850)
Context: It is better to fail in originality, than to succeed in imitation. He who has never failed somewhere, that man can not be great. Failure is the true test of greatness.
Context: It is better to fail in originality, than to succeed in imitation. He who has never failed somewhere, that man can not be great. Failure is the true test of greatness. And if it be said, that continual success is a proof that a man wisely knows his powers, — it is only to be added, that, in that case, he knows them to be small. Let us believe it, then, once for all, that there is no hope for us in these smooth pleasing writers that know their powers.

Source: Dracula

Source: The Star Wars Trilogy

Variant: Find a place inside where there's joy, and the joy will burn out the pain.

Variant: Do what you gotta do so you can do what you wanna do.

This derives from a folk proverb sometimes attributed to Clementine Paddleford, but in use as an "old proverb" as early as 1908, when Paddeford was only 10 years old.
Misattributed
Source: Eat, Pray, Love

Source: The Witch of Portobello (2007), p. 152.
Context: You are what you believe yourself to be.
Don't be like those people who believe in "positive thinking" and tell themselves that they're loved and strong and capable. You don't need to do that because you know it already. And when you doubt it — which happens, I think, quite often at this stage of evolution — do as I suggested. Instead of trying to prove that you're better than you think, just laugh. Laugh at your worries and insecurities. View your anxieties with humor. It will be difficult at first, but you'll gradually get used to it. Now go back and meet all those people who think you know everything. Convince yourself that they're right, because we all know everything, it's merely a question of believing.
Believe.

Source: On Writing: A Memoir of the Craft

Often attributed to Eleanor Roosevelt without an original source in her writings, for example in the introduction to It Seems to Me : Selected Letters of Eleanor Roosevelt (2001) by Leonard C. Schlup and Donald W. Whisenhunt, p. 2 http://books.google.com/books?id=UeFWjTMcLZYC&lpg=PP1&pg=PA2#v=onepage&q&f=false. But archivists have not been able to find the quote in any of her writings, see the comment from Ralph Keyes in The Quote Verifier above.
Disputed

Source: The Diary of a Young Girl

As quoted in Weird Ideas That Work : 11 1/2 practices for promoting, managing, and sustaining innovation (2001) by Robert I. Sutton, p. 95

This, and variants of it, have been been widely circulated as a Quaker saying since at least 1869, and attributed to Grellet since at least 1893. W. Gurney Benham in Benham's Book of Quotations, Proverbs, and Household Words (1907) states that though sometimes attributed to others, "there seems to be some authority in favor of Stephen Grellet being the author, but the passage does not appear in any of his printed works." It appears to have been published as an anonymous proverb at least as early as 1859, when it appeared in Household Words : A Weekly Journal.
It has also often become attributed to the more famous Quaker William Penn, as well as others including Mahatma Gandhi and Ralph Waldo Emerson.
Variants:
I expect to pass through this world but once. If, therefore, there be any kindness I can show, or any good thing I can do any fellow human being let me do it now. Let me not defer or neglect it, for I will not pass this way again.
Writing of an unnamed Quaker, as quoted in Scott's Monthly Magazine Vol. VII, No. 6 (June 1869, p. 475, edited by William J. Scott
I expect to pass through this world but once. Any good thing, therefore, that I can do or any kindness I can show to any fellow human being let me do it now. Let me not defer nor neglect it, for I shall not pass this way again.
As quoted anonymously in Hour by Hour; or, The Christian's Daily Life (1885), compiled by E.A.L., p. 37, and as "the old Quaker's words" in The Unitarian Vol. VI (July 1891); this version was given the title "Do It Now" in Heart Throbs: In Prose and Verse (1905) by Joe Mitchell Chapple.
I shall pass through this world but once! Any good thing, therefore, that I can do or any kindness that I can show to any human being, let me do it now, in his name, and for his sake! Let me not defer or neglect it, for I shall not pass this way again.
Anonymous quotation on a card, as quoted in The Friend, Vol. 61 (1888) by The Society of Friends, p. 364
I shall pass through this world but once. Any good, therefore, that I can do or any kindness I can show to any human being, let me do it now. Let me not defer or neglect it, for I shall not pass this way again.
Anonymous quotation on a card, as quoted in A Memorial of a True Life : A Biography of Hugh McAllister Beaver (1898) by Robert Elliott Speer, p. 169
I expect to pass through this world but once. If, therefore, there be any kindness I can show, or any good thing I can do, to any fellow being let me do it now. Let me not defer nor neglect it, for I shall not pass this way again.
As quoted anonymously in The Lamp Vol. XXVI (February-July 1903)
Disputed

15 January, 1849. As quoted in Elizabeth Gaskell The life of Charlotte Brontë (1870), p. 285

The Lion and the Mouse.
Variant: No act of kindness, no matter how small, is ever wasted.

As reported by Quoteinvestigator on January 11, 2011 http://quoteinvestigator.com/2011/01/11/what-lies-within/ the quote appeared in “Meditations in Wall Street” (1940) by Wall Street trader Henry Stanley Haskins, "a Wall Street trader with a checkered background. The phrase was misattributed because the true author's name was initially withheld. In addition, the assignment of the maxim to a more prestigious individual, e.g., Emerson or Thoreau, made it more attractive and more believable as a nugget of wisdom." Emerson made a number of similar statements — in "The American Scholar," for example, he says "Give me insight into to-day, and you may have the antique and future worlds" — which probably increased the likelihood of misattribution.
Misattributed
Variant: What lies behind us and what lies ahead of us are tiny matters compared to what lies within us.
Variant: What lies behind us and what lies before us are tiny matters compared to what lies within us.

Variant: I learned this, at least, by my experiment: that if one advances confidently in the direction of his dreams, and endeavors to live the life which he has imagined, he will meet with a success unexpected in common hours.
Source: Walden: Or, Life in the Woods

Pearls of Wisdom
Source: Meditation and Its Methods According to Swami Vivekananda

[C]'est la vraie générosité ; vous donnez tout et rien ne semble jamais vous coûter.
All Men are Mortal (1946)

As quoted in The Ring of Truth (2004) by Joseph O'Day

Anonymous advertising copywriter for Menards chain of hardware stores (2000), according to Quote Investigator http://quoteinvestigator.com/2014/01/20/be-yourself
Misattributed

As translated by Katharine Lyttelton, in Joubert : A Selection from His Thoughts (1899)
Source: Striking Thoughts (2000), p. 121; this likely derives from the observation of Joseph Joubert: The goal is not always meant to be reached, but to serve as a mark for our aim.

Letter to Isham Reavis (5 November 1855)
1850s
Context: If you are resolutely determined to make a lawyer of yourself, the thing is more than half done already. It is but a small matter whether you read with anyone or not. I did not read with anyone. Get the books, and read and study them till you understand them in their principal features; and that is the main thing. It is of no consequence to be in a large town while you are reading. I read at New Salem, which never had three hundred people living in it. The books, and your capacity for understanding them, are just the same in all places.... Always bear in mind that your own resolution to succeed, is more important than any other one thing.

Source: Snow

Also found with the alternative spelling: Everything is okay in the end. If it's not okay, it's not the end
Found anonymously on Usenet in 2000 https://groups.google.com/forum/message/raw?msg=alt.support.divorce/gKiyfcAYreo/jjuc6KTu_NAJ. First known attribution to Lennon is from 2011 https://groups.google.com/forum/message/raw?msg=stow-ma-apple-barn/45MNk9KiGsY/vaq6pr8hgI0J.
Disputed
Variant: Everything is OK in the end. If it's not OK, it's not the end.

Mildred Pierce

Source: My Sister's Keeper

Dale Carnegie, quoted in Permission to Play : Taking Time to Renew Your Smile (2003) by Jill Murphy Long, p. 69

Works and Days
1870s, Society and Solitude (1870)

Source: My Sister's Keeper

Source: My Early Life, 1874-1904

Lord Darlington, Act III
Source: Lady Windermere's Fan (1892)

"Proclamation 3560 — Thanksgiving Day, 1963" (5 November 1963) http://www.presidency.ucsb.edu/ws/?pid=9511<!-- Online by Gerhard Peters and John T. Woolley, The American Presidency Project -->
1963
Context: Today we give our thanks, most of all, for the ideals of honor and faith we inherit from our forefathers —  for the decency of purpose, steadfastness of resolve and strength of will, for the courage and the humility, which they possessed and which we must seek every day to emulate. As we express our gratitude, we must never forget that the highest appreciation is not to utter words but to live by them.
Let us therefore proclaim our gratitude to Providence for manifold blessings — let us be humbly thankful for inherited ideals — and let us resolve to share those blessings and those ideals with our fellow human beings throughout the world.

Variant: Even if you are on the right track, you will get run over if you just sit there.

Source: The Secret Garden

Optimism (1903)
Variant: Optimism is the faith that leads to achievement

No known source in Emerson's works; first found as a piece of anonymous folk-wisdom in a 1936 newspaper column:
: Every minute you are angry, you lose 60 seconds of happiness.
:* Junius, "Office Cat" https://www.newspapers.com/newspage/85995624/, The Daily Freeman [Kingston, NY] (30 December 1936), p. 6
Misattributed

Source: 1930s, The Conquest of Happiness (1930)

Often misquoted as: "I have found that most people are about as happy as they make up their minds to be." or "People are just as happy as they make up their minds to be."
This quote is not found in the various Lincoln sources which can be searched online (e.g. Gutenberg). Niether does Lincoln appear more generally to use the phrase "making up {one's} mind". The saying was first quoted, ascribed to Lincoln but with no source given, in 1914 by Frank Crane and several times subsequently by him in altered versions. It was later quoted in How to Get What You Want (1917) by Orison Swett Marden (Thomas Y. Crowell Company, 1917), 74, again without source. Alternative versions quoted are: "I have found that most people are about as happy as they make up their minds to be" and "People are just as happy as they make up their minds to be."


Source: https://quoteinvestigator.com/2012/10/20/happy-minds/#:~:text=%E2%80%9CPeople%20are%20about%20as%20happy,up%20their%20minds%20to%20be.%E2%80%9D&text=Remember%20Lincoln's%20saying%20that%20%E2%80%9Cfolks,up%20their%20minds%20to%20be.%E2%80%9D

Curiously in later books Crane, e.g. Four Minute Essays, 1919, Adventures in Common Sense, 1920, "21", 1930, Crane mentions other routes to happiness and does not again use this quote.

Marden used a great many quotes in his writings, without giving sources. Whilst sources for many of the quotes can be found, this is not true for all. For instance he mentions another story in which Lincoln says "Madam, you have not a peg to hang your case on"; this also does not seem to found in Lincoln sources.

Source: Battlefield of the Mind: Winning the Battle in Your Mind

45
The Gardener http://www.spiritualbee.com/love-poems-by-tagore/ (1915)

Source: Letter to My Daughter