Losing my best friend! (Venting)

My Friend broke it off with his Fiance about five months ago! We were always really close, but after their break up we became closer and I made the mistake of falling in love with him. We have the best relationship ever. We can talk about anything, share anything, give each other the best advice etc. he was going through a really difficult time and I was there for him in anyway you could imagine. When they were together she didn't do anything for him. I cooked for him and did things that a fiancé is supposed to do, because he needed help, and she didn't even care to take care of him (reason he broke it off). We started an intimate relationship shortly after his break-up. Yes I knew I was playing with fire because he really cared about her and I had a feeling they would get back together, but no matter how much you know something will happen, it never prepares you for that moment. Well that moment came today and I am devastated. My heart is so broken. I know we can't remain friends after having had a sexual relationship, and me having strong feelings for him, but more than anything I just need my friend. He's the person I usually go to with all my issues. I have no where else to turn. I want him to be happy and I wish them the best, but how can my misery be someone else's happiness? It just doesn't seem fair! (Aware that I brought this on myself), but it still hurts like hell! Can't help who you fall in love with!