“Life isn’t like coursework, baby. It’s one damn essay crisis after another.”

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

"Law and Freedom in History," <i>A Study of History</i>, Vol. 2 (1957). The embedded quotation is attributable to Elbert Hubbard.

Source: Bishops Want Catholic Baby Boom To Halt Slide In Kerala Christian Community https://www.ucanews.com/story-archive/?post_name=/2007/09/14/bishops-want-catholic-baby-boom-to-halt-slide-in-kerala-christian-community&post_id=6428 (September 13, 2007)

God Bless You, Mr. Rosewater (1965)
Context: Hello, babies. Welcome to Earth. It's hot in the summer and cold in the winter. It's round and wet and crowded. At the outside, babies, you've got about a hundred years here. There's only one rule that I know of, babies — "God damn it, you've got to be kind."

The Plain Dealer (1677), Prologue.

Source: Lit From Within: Tending Your Soul For Lifelong Beauty

Source: Stone of Tears

Light (1919), Ch. VII - A Summary
Context: One after another, sundry women have occupied my life. Antonia Veron was first. Her marriage and mine, their hindrance and restriction, threw us back upon each other as of yore. We found ourselves alone one day in my house — where nothing ever used to happen, and she offered me her lips, irresistibly. The appeal of her sensuality was answered by mine, then, and often later. But the pleasure constantly restored, which impelled me towards her, always ended in dismal enlightenments. She remained a capricious and baffling egotist, and when I came away from her house across the dark suburb among a host of beings vanishing, like myself, I only brought away the memory of her nervous and irritating laugh, and that new wrinkle which clung to her mouth like an implement.
Then younger desires destroyed the old, and gallant adventures begot one another. It is all over with this one and that one whom I adored. When I see them again, I wonder that I can say, at one and the same time, of a being who has not changed, "How I loved her!" and, "How I have ceased to love her!"

Source: The Happy Prince and Other Tales