Heartbreaking quotes
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“my feet will want to walk to where you are sleeping
but
I shall go on living.”
Source: Where the Red Fern Grows

“Sometimes the only way the good Lord can get into some hearts is to break them.”
Source: Sugar Daddy
“When trees burn, they leave the smell of heartbreak in the air.”
Source: Welcome to Harmony

“True friends never apart maybe in distance but never in heart”
“There is a thin line that separates laughter and pain, comedy and tragedy, humor and hurt.”
“it's wrong to be right; it's right to be wrong.”
“Insanity is doing the same thing over and over again but expecting different results.”
Brown did include this quote in her book Sudden Death (Bantam Books, New York, 1983), p. 68, but it appears she was just paraphrasing a quote that had already been written elsewhere. The earliest known appearance of a similar quote is the "approval version" of the Narcotics Anonymous "Basic Text" released in November 1981, which included the quote "Insanity is repeating the same mistakes and expecting different results." A PDF scan of the 1981 approval version can be found here http://www.nauca.us/wp-content/uploads/2015/04/1981-11-Basic-Text-Approval-Form-White.pdf, with the quote appearing on p. 11 (p. 25 of the PDF), at the end of the fourth paragraph (which begins "We have a disease; progressive, incurable and fatal"). More in this article https://quoteinvestigator.com/2017/03/23/same/ on Quote Investigator website.
Misattributed

“In the absence of love, there is nothing worth fighting for.”
Quoted in M. Kumar, Dictionary of Quotations Page 136 http://books.google.co.uk/books?id=N0VKD37eY94C&pg=PA136&dq=%22In+the+absence+of+love,+there+is+nothing+worth+fighting+for%22&hl=en&sa=X&ei=7ns3T6XuNI6n8gOnpaWqAg&ved=0CDEQ6AEwAA#v=onepage&q=%22In%20the%20absence%20of%20love%2C%20there%20is%20nothing%20worth%20fighting%20for%22&f=false

“Nothing doth more hurt in a state than that cunning men pass for wise.”
Of Cunning
Essays (1625)

“A divorce is like an amputation; you survive, but there’s less of you.”
Time magazine (19 March 1973)
Source: Life, the Truth, and Being Free (2010), p. 135

“Of all the aspects of social misery nothing is so heartbreaking as unemployment …”
Source: Twenty Years at Hull-House (1910), Ch. 10
Some Men are More Perfect Than Others (1973)

“The less you think, the better it is.”
1960's, Conversations with Samuel Beckett and Bram van Velde' (1965 - 1969)

“Sometimes, only one person is missing, and the whole world seems depopulated.”
"L'Isolement", Méditations Poétiques (1820)

Diary entry (18 August 1908), quoted in The Later Years of Thomas Hardy (1930), by Florence Emily Hardy, ch. 10, p. 133

“This was such bad writing that it was good.”
The Daring Young Man on the Flying Trapeze (1934), A Cold Day

“The bigger they are, the harder they fall.”
Some sources claim it was said first by Walcott http://coxscorner.tripod.com/walcott.html, but the English version of this ancient proverb is generally attributed to Bob Fitzsimmons, as documented in the Oxford Dictionary of Proverbs ( 6th edition, 2015, p. 26 https://books.google.com/books?id=LMGPCgAAQBAJ&pg=PA26).
Disputed

“For of all sad words of tongue or pen,
The saddest are these: "It might have been!"”
Bret Harte wrote a famous parody of this famous poem, "Mrs. Judge Jenkins" in which the Judge marries Maud, and which he ends with the lines:
Maud soon thought the Judge a bore,
With all his learning and all his lore;
And the Judge would have bartered Maud's fair face
For more refinement and social grace.
If, of all words of tongue and pen,
The saddest are, "It might have been,"
More sad are these we daily see:
"It is, but hadn't ought to be".
Maud Muller (1856)
Context: Alas for maiden, alas for Judge,
For rich repiner and household drudge!
God pity them both! and pity us all,
Who vainly the dreams of youth recall;
For of all sad words of tongue or pen,
The saddest are these: "It might have been!"

The Clod and the Pebble, st. 1
1790s, Songs of Experience (1794)