I'm in love with someone who loves another., ? Options

[bambamboom]

I love a guy that has a girlfriend. They been going out for years and he loves her. I met him at college. He is in all of my classes since he is taking up Nursing as well. We study a lot together and the tension is there between us. I didn't know he had a girlfriend when we were talking. I was really into him. Then he dropped the line and told me he had a girlfriend. Everytime we are with each other it feels right to both of us. He says he likes me but he can't because of his girlfriend and what they have gone through, he can't imagine not living without her. We were together studying one day and our feelings for each other just went overboard. We had sex. I felt really gulity knowing I had sex with a guy who has a girlfriend. He says that he doesn't really care about it and that "it's college". It makes me sad to think that I feel like a bootycall...He takes his girlfriend seriously and not me. I love this guy. He doesn't even know. He says he likes me in a way that I'm gorgeous but I don't just want to be his friend.
idk
what should I do?