Advice on staying friends with an ex?

My boyfriend of 4 months & i just broke up but i really want to try to still be friends. he was my bestfriend as well, and i miss him. he said he wants to try to be friends too. but this is what makes it difficult: he now likes this other girl, who he danced with at a social while we were going out, after telling me he wasn't going to dance with any girls. then he started liking her & dumped me because of it i guess. i don't know the whole story. he said he was really sorry though, and i belive him. he is really nice and is not the type to do anything on purpose or with the intention of hurting someone. my guess is he danced with her because it was a couples dance & he just asked a friend/she asked him & it ended up being more than he thought. which is understandable. but i'm going to see him up at the rink (i skate with his sister, that's how we met) & i don't want things to be awkward, i still want him as my bestfriend. help pleaseee!