How to I stop feeling bad about something I did in the past?

You could try to talk to someone about it. That usually helps. It really depends on what the act was. In any case though, I think it comes down to the consequences. If you've already dealt with those then it's best to learn from your mistakes and move on. And if not, then maybe it's time to. Guilt is usually remedied with a clear conscience. 

Learn from your mistakes and don't do it again. See if there is a way you can bring restitution to those you have hurt with your actions. But most of all you got to learn how to forgive yourself.

Focus on getting it right, now, as you move through your life. Dwelling on the past is just spinning your wheels. Life is too short.

If you did something that hurt someone, you apologize to them.

Perhaps it is just "unnecessary" guilt that you are feeling.

Re-evaluate just to be sure.