Of all human emotions, regret has to be up there as the most useless.
It is paralyzing, depressing, and serves no purpose.
It leads to rear-view-mirror-living, self-pity, and a general throwing up of hands.
Introspection is better as you learn something from it. Being self-critical can help to lead you to fix the things that bothered you. Acceptance of misfortune has an element of peace, healing, and moving-on. Being sorry for a misdeed leads you to apologizing others you might have hurt. Even giving-up has an element of closure and moving on to the next thing.
But regret, by itself, has no healing power. It keeps you in the fantasy of what-could-have-been; while what-can-still-be slips away.