Remember...

To be nice to yourself; we all do things we wish that we hadn't and have regrets, but each regret that you have is replacing what could otherwise be a moment of happiness.  If you can say you're sorry, do so; if what you did can't be addressed by talking to someone, then let it go, and every time you think about the misdeed, say to yourself, "Let it go."  Remember something that you did well instead or concentrate on doing something good in the present.