Regret is a catch twenty2.
It’s said that you shouldnt regret anything because it has shaped you to become the person you are today.
But if we didnt feel the need or want to change something that had occured in the past then there would be no reason for the word regret to have ever existed.
Therefor I believe there are instances where it would be considered appropriate to feel regret.

Accepting truths can suck
lying to yourself sucks too