“One should accept who they are or make changes to meet their expectations, instead of trying to be what they are not.”
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“Change what cannot be accepted and accept what cannot be changed.”

Niebuhr's preferred form, as declared by his widow
The Serenity Prayer (c. 1942)
Variant: God, give us grace to accept with serenity the things that cannot be changed, courage to change the things which should be changed, and the wisdom to distinguish the one from the other.

Source: Kavanagh: A Tale (1849), Chapter 13.
Context: Ah, how wonderful is the advent of spring! — the great annual miracle of the blossoming of Aaron's rod, repeated on myriads and myriads of branches! — the gentle progression and growth of herbs, flowers, trees, — gentle and yet irrepressible, — which no force can stay, no violence restrain, like love, that wins its way and cannot be withstood by any human power, because itself is divine power. If spring came but once in a century, instead of once a year, or burst forth with the sound of an earthquake, and not in silence, what wonder and expectation there would be in all hearts to behold the miraculous change! But now the silent succession suggests nothing but necessity. To most men only the cessation of the miracle would be miraculous and the perpetual exercise of God's power seems less wonderful than its withdrawal would be.