Answer by Christopher Lamke:
The Moving Finger writes; and, having writ,
Moves on: nor all your Piety nor Wit
Shall lure it back to cancel half a Line,
Nor all your Tears wash out a Word of it.
This is from the Edward Fitzgerald translation of the Rubaiyat of Omar Khayyam. Fitzgerald took some liberties with the literal meaning of the Arabic here and elsewhere, but the results are brilliant.
I made many mistakes and hurt many people in my early adult life, and this verse (among many in the Rubaiyat) helped me to realize the importance of empathy and wisdom and the impossibility of ever truly making up for past wrongs. In short, the idea encapsulated in this verse helped me to grow up.
I try to follow this ideal in daily life, though I come nowhere near "fully and completely" following it as the question requires.