I think the above comment says it all. Sadly this man is using you and you are allowing yourself to be used. Where is your self-respect? How would you feel if he were cheating on you with another girl as he now cheats on his girl with you? Do you honestly think he'd be faithful to you if he plays around on this girl? Do you expect her to be happy when she finds her boyfriend cheating on her with you?
Even if she really is a controller; it's up to him to have courage& leave her for himself not for you. It's not an option for him to treat both her and you with disrespect. You are also acting badly to this woman has not done anything to you that would incur such behaviour.