A feeling of sadness, repentance, or disappointment over something that has happened or been done defines regret. It can also be something that was left undone. Regret is always there, first in line, to occupy our mind. No matter if you seemingly did everything right, regret will find an opening.
Letting go of regret is easier said than done. As a bell cannot be un-rung, regret, so often, cannot be undone. One’s only hope is to accept what you cannot change and forgive yourself. That, too, is easier said than done. The irony is that if you don’t forgive yourself, that omission will itself become a regret.
Brethren, I count not myself to have apprehended: but [this] one thing [I do], forgetting those things which are behind, and reaching forth unto those things which are before, (Philippians 3:13)