Consider the people with whom you've cheated (without judging yourself); what was on your mind at the time (how did you program yourself to do this thing that it seems that you don't want to do)? What was it about them? Can you talk to your partner about what you need - knowing she can't fix you but can work with you to deal with this?

I don't think sexual assault in your past is the (sole) reason you cheat. Not everyone who suffers abuse perpetrates it on others. You appear to not have both feet pointed in your partner's direction, so you have not truly invested your all in the relationship. The only way to know
why you cheat' is to truly look within. Journal it. Talk it out in therapy. Most likely there is not one concrete reason. Being afraid to be hurt could be part, but you are also knowingly inflicting hurt on someone you say you love. Think about past patterns, consider your history. Have you ever been faithful in any of your relationships. Lots ot think about!