“It was one thing, after all, to know his feelings for Amanda hadn't changed; it was another thing entirely to face the future with the certainty that they never would.”

Apology for the Abbé de Prades (1752)

Source: The 9 Steps to Financial Freedom: Practical and Spiritual Steps So You Can Stop Worrying

X, 11
Meditations (c. 121–180 AD), Book X
Context: Acquire the contemplative way of seeing how all things change into one another, and constantly attend to it, and exercise thyself about this part [of philosophy]. For nothing is so much adapted to produce magnanimity.... But as to what any man shall say or think about him, or do against him, he never even thinks of it, being himself contented with these two things: with acting justly in what he now does, and being satisfied with what is now assigned to him; and he lays aside all distracting and busy pursuits, and desires nothing else than to accomplish the straight course through the law, and by accomplishing the straight course to follow God.

Source: The God of Small Things

1950s, Unpopular Essays (1950)

From an October 1930 letter to Arthur Davison Ficke, as variously described by her biographers, e.g.:
[L]ife was not so much "one damn thing after another" as "one damn thing over and over"
As paraphrased ("she had sent [...] a half-comic note, complaining that...") with quoted phrases in Jean Gould, The Poet and Her Book: A Biography of Edna St. Vincent Millay (1969), p. 198
[L]ife isn't one thing after another, it's the same thing over and over
As paraphrased ("she writes that...") and apparently Bowlderized in Miriam Gurko, Restless spirit: the life of Edna St. Vincent Millay (1962), p. 197
[I]t was not true that life is one damn thing after another — it was one damn thing over and over
As paraphrased ("Edna had written [...] that...") in Joan Dash, A Life of One's Own: Three Gifted Women and the Men they Married (1973), p. 189
The paraphrase by Dash appears to be the origin of later popularly attributed variants, e.g.:
It is not true that life is one damn thing after another. It's the same damn thing over and over.
As attributed without citation in Psychoanalysis Today: A Case Book (1991) by Elizabeth Thorne and Shirley Herscovitch Schaye, p. 93
It is not true that life is one damn thing after another. It's the same dang thing over and over again.
As attributed without citation in The Last Word: A Treasury of Women's Quotes (1992) by Carolyn Warner

Source: Just Listen

Source: The Happy Prince and Other Tales