“When you've suffered a great deal in life, each additional pain is both unbearable and trifling.”

Source: Out of Sheer Rage: Wrestling With D.H. Lawrence

Source: Why Men Marry Bitches: A Woman's Guide to Winning Her Man's Heart

Vorkosigan Saga, Barrayar (1991)
Context: But pain... seems to me an insufficient reason not to embrace life. Being dead is quite painless. Pain, like time, is going to come on regardless. Question is, what glorious moments can you win from life in addition to the pain?

Finding Life after Death
What About the Big Stuff (2002)

"When I Think Of My People Broken Down", as translated in "The Poetry of Sri Lanka", in Journal of South Asian Literature, Vol. 12, No. 1 (Fall-Winter 1976), published by Asian Studies Center, Michigan State University, p. 11
Context: Unbearable becomes the pain in my heart — 
When I think of my people, broken down,
broken by disease in mind and limb. On the edge of life they always linger;
For countless are the diseases
Of Ignorance and Hunger. And on treacherous paths to Slavery
like children blind, they would walk behind
strangers from over the sea. O, divine Land, blessed by the gods!
O, ancient Mother of Culture and Art!
Thy children today are spineless hordes.

1960s, The Autobiography of Bertrand Russell (1967-1969)
Context: Three passions, simple but overwhelmingly strong, have governed my life: the longing for love, the search for knowledge, and unbearable pity for the suffering of mankind. These passions, like great winds, have blown me hither and thither, in a wayward course, over a deep ocean of anguish, reaching to the very verge of despair.

Denn zu einem großen Manne gehört beides: Kleinigkeiten als Kleinigkeiten, und wichtige Dinge als wichtige Dinge zu behandeln.
Gotthold Ephraim Lessing, Hamburgische Dramaturgie (1767 - 1769), Vierunddreißigstes Stück Den 25. August 1767 http://www.gutenberg.org/files/10055/10055-8.txt
Misattributed