How can I get my friends to forget my promiscuous past?

Your reputation is in shambles. That is part of the many consequences due to your poor choices. There's no way to erase the past, but you can change your image by NOT SLEEPING AROUND . . . . . . . PERIOD!!!! I know you say you don't do it anymore, but they don't know that. It will take a long time for them to look at you in a new light. Why not join a church youth group or a charity volunteer group and get to work cleaning up your reputation. Hopefully, when they see you do not respond to their cruel teasing, they will stop.