Regret... what is it good for?

Absolutely nothing. Neither is guilt. There are people in life who want to just shove past events under you nose and try to force you to regret them and feel guilty. You may feel it, because we have no control over our feelings, they just happen. But you choose how to deal with the regret and guilt. You can choose to dwell on it and stunt your own growth and not fully live your own life, or you can choose to work your way through it - as a buddhist would say: Look at the feeling, examine it, feel it and then throw it away.

In terms of those people who shove things under your nose - shove them out of your life. You will feel so much better for having done so. If you don't want to shove them completely out, then compartmentalize how you feel about that person and how you deal with them. Don't let them shove that stuff under your nose - if they try to and won't stop when you ask them to, walk away and let them know that when they feel like talking to you without doing those things that you would be more than happy to see them or talk to them. There is so much negativity in this world, blocking what you can will be the best thing for you in the long run. This is your life - go on and live it the way that you want to! Be at peace!