I've cheated on my partner a few times before in the past, but now I want to stop because she says she will leave me if I don't. I don't want to do it again, but I know my weaknesses, so I'm not sure if I can really stop. How can I stop cheating on her?

Know the reason why you are cheating. Is it because of the physical appearance of the other party or is it because they give you time etc? Once you know the root cause, you can start addressing them. Then talk to your partner and discuss how can she help also help you to totally overcome cheating.