Look, buddy...I'm probably going to come off as a bitch here, but you need a bit of the cold, hard truth.
You attract what you project.
If you're constantly approaching dating situations with that pessimistic, self-pitying, negative "I always get cheated on" attitude - it's gonna show in your behavior. The more you obsess over past experiences of being cheated on, get yourself more paranoid about it and fall into the mindset that "ALL women will cheat on you" - the more you're going to appear insecure and give off a negative vibe. And most women do not find that attractive. If you obsess over cheating to the point where your girlfriend feels uncomfortable enough with you to have to resort to cheating - then that's your own fault.