It sounds like a very difficult situation for you. Is it possible to not care as much about him? I mean it seems like he's doing what he wants to do but is coming in your room getting clothes and saying that he's going to the shop but then staying out all night. Calling you an asshole

. And that's the stuff that needs to be discussed. "Ground rules and Boundaries". Like if he's going to live his life living with you but seeing someone else, he's gotta respect the fact that you have feelings about all this. To lessen the pain for you.