I guess I don't get why you aren't seeing other people? You want to, he doesn't care. So, go see other people...
I also don't get why you stay. He doesn't love you and never will. He's involved in not one but two completely messed up relationships. He cheats. He wants to be monogamous but completely sucks at it. (Don't think so? Read over your descriptions of his other relationships.) If he wanted to really be monogamous, he would be.
Unselfish love doesn't mean doormat. You deserve a real relationship (or two or three!) with people who love you and want to be with you, not fuck you on the side. You deserve better.