This is going to stand in the way of you ever finding someone for yourself. I think he is treating you like a sister. I know you want cheering up but the only way you are going to do this is move on. Never mind not being fair on his GF its not fair on you.
I think this guy is playing with your feelings. Why not go out and get yourself a boyfriend (or even make one up, if you cant bear to) and see what his reaction would be........... Talk about your new b/f to him all the time and if he gets a bit jealous, then youl know.......................