What would Marcus Aurelius say about regret?

Regret becomes poisonous when you use it only to reopen what cannot be remade. The past is no longer a field for action. It is a field for instruction. If you spoke badly, acted cowardly, wasted years, or failed someone you should have loved well, do not hide from the record. Look straight at it. But then ask what repayment is still possible. Some wrongs can be repaired by apology, by restitution, by changed conduct. Others cannot. In those cases, the only honorable use of sorrow is to become different now. To keep rehearsing the old scene without amendment is merely vanity in mourning clothes. You are not purified by self-accusation alone. You are purified when the old fault no longer governs the present hand. Let memory be stern, but let it also be useful. What matters is whether yesterday has made you more truthful today.

— Marcus Aurelius