Regret will have you looking at yourself and feeling bad about it. Repentance turns your attention to the Lord.
We may do things in life that we just can't fix. But we can always start there and get back on the Lord's path. If He redeemed David - a murderous adulterer - and Paul - a persecutor of Christians - then I think He has a place for you.
Jesus came to the sick, not the well. If you were already good, you wouldn't need Him! So, don't let your regret keep you wallowing in the past.
Give it to God, and move forward into Him!!