How can I stop thinking of something negative I’ve done in the past?

It depends on what feeling or what emotion the supposedly negative thing that you have done is creating in you. If it is the emotion of guilt then this is telling you that you have done something that goes against your true self. So you learn something about yourself and you learn the lesson. Don’t do it again, move on. This is a learning planet. We are meant to learn from our experiences, not beat ourselves up over them.