greatest mistake of one's life
A dramatic set phrase used when someone makes an irreparable, deeply regrettable blunder that will haunt them forever. Often used in self-reproach or to describe a catastrophic error.
Making such a simple mistake is the greatest blunder of my life.
He regrets that decision as the worst mistake of his life.
A set phrase combining 一生 (one's whole life) and 不覚 (failure, blunder). The exact origin is uncertain, but it has been used in Japanese literature and speech to express a mistake so severe it defines one's life.