While I was in Kopan monastery, we received teachings from Ven Thubten Pema. She is an Australian non who spends usually 3 months a year in Kopan, Nepal to teach.
Regret is the recognition of having engaged in harmful actions or mistakes and it is a sincere feeling. You do not want to do it again.
Guilt is self condemnation, feelings of shame focusing on personal unworthiness.
They are different energies.
Regret is good, it makes us want to change. Guilt is useless. In Tibetan it does not even exist the word. It brings you nothing.