How do I deal with having feelings for a friend?

I have feelings for one of my closest friends. I really thought I'd gotten over him, but clearly I haven't. He has a history of being very casual with girls, but then he seemed seriously interested in one girl, although now he seems to be interested in a different one. . . He at least definitely does not seem interested in me. We did actually sleep together once before we were at all close and when I had no feelings for him (we're 20 and in college). I know he does find me attractive and cares about me as a friend, but I'm pretty sure this is as far as it's ever going to go. What do I do? How do I get over it? His friendship is very important to me, but I can't stop being sad when I'm around him sometimes.