Youthful follies I: what did you learn from some major mistake?

“All of us who are worth anything, spend our manhood in unlearning the follies, or expiating the mistakes of our youth.”
—Percy Bysshe Shelley

Tell us about a great and valuable lesson you learned from some youthful error.

(I can’t help noting that Shelley, who died at age 29, must have been one fast hand at expiating if he was on to that stage while most of us are still busy racking up the follies.)

——-

Companion question here.