If your feelings for him are too strong, and you feel like you wanna be with him every time you're with him...I think it's best if you try to move on an forget about him. Hard as it may sound, that sounds like the best option if they're uncontrollable, cause he's got someone else. You should let him know that you don't know if you can be friends because of this and see what he says. If you know you can keep it under control, and think you can be friends with him, then go for it. Who knows, someone else might come your way.
I had somethin like that happen to me. A boy i went out with cheated on me. Kinda hurt, did the whole cryin thing...can't b bothered to do anythin....etc etc and then couple months down the line, he wants to be friends, and says that he likes my company etc. I couldn't figure out whether I still had feelings for him or not though. He's got a nu girl, that apparently don't feel comfortable with me bein friends with him....lol that just made me wanna be friends with him even more lol. Just when I thought I was startin to have feelings for him again, I found someone else lol..well..someone found me anyway. So maybe you'll find someone else (i know you're thinkin it's unlikely) but i thought that once, but I got a nu one 2 worry about lol, andnow ma ex islong forgotten, I realised I don't wanna b friends with a boi that took advantage of me and cheated on me an didn't feel no guilt for what he did, an then havin the cheek to ask me to be friends?? What was I thinkin when I agreed to bein friends with him?!? But that's what worked for me, when I realised that I didn't need him in my life, cause he jus bought back bad memories. But if you think you can deal with it, then be friends with him, and try not to give in!! Juss say you two did get back together....chances are he'd cheat on you again sometime down the line, so it's either friendship and keeping your feelings under control, or u 4get all bout him

, hope that helped