Life quotes

Attributed on the internet but not found in print prior to an attribution in Aero Digest, Vols. 58–59, 1949, p. 115 https://books.google.com/books?id=q2ofAQAAMAAJ&dq=%22Life+is+simple%22+but+we+insist+on+making+it+complicated&focus=searchwithinvolume&q=%22Life+is+simple%22+
Misattributed, Not Chinese

Variant: Some people never go crazy. What truly horrible lives they must lead.

Could you be loved
Uprising (1979)
Variant: I know that I'm not perfect and that I don't claim to be,
so before you point your fingers make sure your hands are clean.
Context: Who are you to judge the life I live?
I know that I'm not perfect and that I don't claim to be,
so before you point your fingers make sure your hands are clean.

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 The Harper Book of Quotations (1993) edited by Robert I. Fitzhenry, p. 261
General sources
Variant: In three words I can sum up everything I've learned about life: it goes on.

Source: A Thousand & One Epigrams: Selected from the Writings of Elbert Hubbard (1911), p. 74

Source: Jordan, Michael. I Can't Accept Not Trying : Michael Jordan on the Pursuit of Excellence. San Francisco, CA: HarperSanFrancisco, 1994. p. 129
Context: I can accept failure. Everyone fails at something. But I can't accept not trying [no hard work].
Talent wins games, but teamwork and intelligence wins championships. (p. 20, 24)

As quoted in Journal of France and Germany (1942–1944) by Gilbert Fowler White, in excerpt published in Living with Nature's Extremes: The Life of Gilbert Fowler White (2006) by Robert E. Hinshaw, p. 62. From the context http://books.google.com/books?id=_2qfZRp9SeEC&lpg=PP1&pg=PA62#v=onepage&q&f=false it seems that White did not specify whether he had heard Einstein himself say this or whether he was repeating a quote that had been passed along by someone else, so without a primary source the validity of this quote should be considered questionable.
Some have argued that elsewhere Einstein defined a "miracle" as a type of event he did not believe was possible—Einstein on Religion by Max Jammer (1999) quotes on p. 89 from a 1931 conversation Einstein had with David Reichinstein, where Reichinstein brought up philosopher Arthur Liebert's argument that the indeterminism of quantum mechanics might allow for the possibility of miracles, and Einstein replied that Liebert's argument dealt "with a domain in which lawful rationality [determinism] does not exist. A 'miracle,' however, is an exception from lawfulness; hence, there where lawfulness does not exist, also its exception, i.e., a miracle, cannot exist." ("Dort, wo eine Gesetzmässigkeit nicht vorhanden ist, kann auch ihre Ausnahme, d.h. ein Wunder, nicht existieren." D. Reichenstein, Die Religion der Gebildeten (1941), p. 21). However, it is clear from the context that Einstein was stating only that miracles cannot exist in a domain (quantum mechanics) where lawful rationality does not exist. He did not claim that miracles could never exist in any domain. Indeed, Einstein clearly believed, as seen in many quotations above, that the universe was comprehensible and rational, but he also described this characteristic of the universe as a "miracle". In another example, he is quoted as claiming belief in a God, "Who reveals Himself in the lawful harmony of the world."
As quoted in From Yale to Jail: The Life Story of a Moral Dissenter (1993) by David T. Dellinger, p. 418
Disputed
Variant: There are only two ways to live your life. One is as though nothing is a miracle. The other is as though everything is a miracle.
Variant: There are only two ways to live your life. One is as though nothing is a miracle. The other is as though everything is a miracle.

Variant: Some people want it to happen, some wish it would happen, and others make it happen.

The Soul of Man Under Socialism (1891)
Context: With the abolition of private property, then, we shall have true, beautiful, healthy Individualism. Nobody will waste his life in accumulating things, and the symbols for things. One will live. To live is the rarest thing in the world. Most people exist, that is all.

As quoted in Our Precarious Habitat (1973) by Melvin A. Benarde, p. v

Shared on her Facebook page https://www.facebook.com/MayaAngelou/posts/10150251846629796, July 4, 2011

As quoted in Our Precarious Habitat (1973) by Melvin A. Benarde, p. v
Context: Nothing in life is to be feared, it is only to be understood. Now is the time to understand more, so that we may fear less.

Mildred Pierce

In answer to a question asked by the editors of Youth, a journal of Young Israel of Williamsburg, NY. Quoted in the New York Times, June 20, 1932, pg. 17 http://select.nytimes.com/gst/abstract.html?res=F40617F83B5A13738DDDA90A94DE405B828FF1D3
Unsourced variant: Only a life in the service of others is worth living.
1930s
Variant: I believe in one thing—that only a life lived for others is a life worth living.

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

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/

Source: Going Bovine

Source: I Have a Dream: Writings and Speeches That Changed the World

Variant: The best and most beautiful things in the world cannot be seen or even touched. They must be felt with the heart.
Variant: The best and most beautiful things in the world cannot be seen, nor touched... but are felt in the heart.

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

Attributed quotes

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.

Variant: Life can only be understood going backward, but must be lived going forward.

Source: Winnie-the-Pooh

Ch. IX : Outdoors and Indoors, p. 336; the final statement "quoted by Squire Bill Widener" as well as variants of it, are often misattributed to Roosevelt himself.
Variant: Do what you can, with what you have, where you are.
Attributed to Roosevelt in Conquering an Enemy Called Average (1996) by John L. Mason, Nugget # 8 : The Only Place to Start is Where You Are. <!-- The Military Quotation Book, Revised and Expanded: More than 1,200 of the Best Quotations About War, Leadership, Courage, Victory, and Defeat (2002) by James Charlton -->
Variant: Do what you can, with what you've got, where you are.
Context: There are many kinds of success in life worth having. It is exceedingly interesting and attractive to be a successful business man, or railroad man, or farmer, or a successful lawyer or doctor; or a writer, or a President, or a ranchman, or the colonel of a fighting regiment, or to kill grizzly bears and lions. But for unflagging interest and enjoyment, a household of children, if things go reasonably well, certainly makes all other forms of success and achievement lose their importance by comparison. It may be true that he travels farthest who travels alone; but the goal thus reached is not worth reaching. And as for a life deliberately devoted to pleasure as an end — why, the greatest happiness is the happiness that comes as a by-product of striving to do what must be done, even though sorrow is met in the doing. There is a bit of homely philosophy, quoted by Squire Bill Widener, of Widener's Valley, Virginia, which sums up one's duty in life: "Do what you can, with what you've got, where you are."

Attributed to Monroe in self-help books and on social media, this quotation is of unknown origin and date.
Misattributed

This Business of Living (1935-1950)

Frequently attributed to Nin, but without cited source in her work (possibly due to a quotation in Living on Purpose: Straight Answers to Universal Questions (2000) by Dan Millman that attributed the quote to Nin without source).
In March 2013, a former Director of Public Relations at John F. Kennedy University in Orinda, Elizabeth Appell, claimed she had authored the quote in 1979 for an inspirational header on a class schedule: http://anaisninblog.skybluepress.com/2013/03/who-wrote-risk-is-the-mystery-solved/
Disputed
Variant: The day came when the risk to remain tight in a bud was more painful than the risk it took to blossom.

Quote attributed to Picasso in TIME, October 4, 1976, Modern Living: Ozmosis in Central Park http://quoteinvestigator.com/2015/03/07/child-art/ http://content.time.com/time/magazine/article/0,9171,918412,00.html
Disputed
Variant: All children are artists. The problem is how to remain an artist once he grows up.

Apologia pro Vita Sua http://www.fordham.edu/halsall/mod/newman/apologia1.html (1864).

Source: Pygmalion

Source: The Autobiography of Eleanor Roosevelt

Frequently misattributed to Marilyn Monroe or Kurt Cobain.
Source: https://books.google.com/books?id=xUtdDnEhkMMC&pg=PT12&lpg=PT12#v=onepage&q&f=false
Source: Autumn Leaves, Philosophical eLibrary, 2012, (Feuillets d'automne, 1941, trans. Jeanine Parisier Plottel)

Source: Journals of Ralph Waldo Emerson, with Annotations - 1841-1844

Source: P.S. I Love You

Variant: You build on failure. You use it as a stepping sone. Close the door on the past. You don't try to forget the mistakes, but you don't dwell on it. You don't let it have any of your energy, or any of your time, or any of your space.

Source: The Diary of a Young Girl

Il y a assurément un autre monde, mais il est dans celui-ci...
Œuvres complètes, vol. 1, Gallimard, 1968.

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.

Manuscript Found in Accra (2012), Love has always passed me by

Source: The Diary of Anne Frank

Attributed to Kierkegaard in a number of books, the earliest located on Google Books being the 1976 book Jack Kerouac: Prophet of the New Romanticism by Robert A. Hipkiss, p. 83 http://books.google.com/books?id=g_JaAAAAMAAJ&q=%22problem+to+be+solved%22#search_anchor. In the 1948 The Hibbert Journal: Volumes 46-47 the quote is referred to as "the famous Kierkegaardian slogan" on p. 237 http://books.google.com/books?id=UuDRAAAAMAAJ&q=%22the+famous+Kierkegaardian+slogan+life+is+not+a+problem+to+be+solved%22#search_anchor, which may be intended to suggest the phrase is Kierkegaard-esque rather than being something written by Kierkegaard. In reality this seems to be a slightly altered version of the quote "The mystery of life is not a problem to be solved; it is a reality to be experienced" which appeared in the 1928 book The Conquest of Illusion by Jacobus Johannes Leeuw, p. 9 http://books.google.com/books?id=OFdVAAAAMAAJ&q=%22not+a+problem+to+be+solved%22#search_anchor.
Misattributed

"Personal Conduct" http://books.google.com/books?id=IYOcAQAAQBAJ&q=%22The+stupid+neither+forgive+nor+forget+the+na%C3%AFve+forgive+and+forget+the+wise+forgive+but+do+not+forget%22&pg=PA177#v=onepage, p. 51. http://openlibrary.org/works/OL15151528W/The_Second_Sin
The Second Sin (1973)

Variant: Never let your schooling interfere with your education.

Attributed to Winston Churchill in The Prodigal Project : Book I : Genesis (2003) by Ken Abraham and Daniel Hart, p. 224 and other places, though no source attribution is given. It actually derives from an advertising campaign for Budweiser beer in the late 1930s.
Misattributed
Variant: Success is not final, failure is not fatal: it is the courage to continue that counts.
Source: http://quoteinvestigator.com/2013/09/03/success-final/

Source: The Monk Who Sold His Ferrari

Source: Tuesdays with Morrie