Still In Love With His Ex- What Should I Do?

I moved in with my mates a few months ago and recently have started to have feelings for one of them. We were'nt particularily close before the move but have found that we have alot in common. We're very tangible with each other and spend most of our time together, holding hands in public etc so to the outside world we look like a couple and it has been commented upon by our flatmates. In private he tells me he loves me and our conversations over the computer when he goes to stay with his parents are very affectionate. All sounds very good HOWEVER he is still very much in love with a girl he used to date. She is now his best mates girlfriend. But he talks about her all the time, which was fine when we were just friends but as we are now it really upsets me. I know its difficult to get over someone you loved and have experienced that myself but is it worth getting into a full blown relationship in the hope that these feelings will subside? I would be grateful for any advice! x