How should I handle this situation?

About half a ano atrás there was a guy I always saw in weekends at my regular bar. We have a lot of mutual friends, and we became friends too. One night at the bar, things got really flirty between us, and at the end of the night we kissed. The seguinte couple of weeks we continued doing that. Everything was pretty casual. After a few weeks I wanted to end this 'fling' with him, because I wasn't in amor with him, and toughed I wasn't going to be, so it seemed better to stop. It felt like the right thing to do. A few months later things got flirty again, but then I heard he had a girlfriend now. The last couple of months we have been just friends. Now here's my problem: I now fell in amor with him. Every Friday and Saturday night he spends much mais time with me than with his girlfriend, but in the end, he goes início with her. It kills me. I would never kiss a guy who has a girlfriend, and he would never cheat. He doesn't know how I feel, but I don't really want to tell him, because I don't want to lose him as a friend. Does anybody have any idea on how I should handle this situation?