If he's not going to give a second chance then there's not much to do.
I think that its not really about chances really, more about willing to talk over problems and work with the relationship. But in order for that to happen you need to have someone with enough guts to clearly state the problem, the receiver needs to be mature enough to realize its not an attack and accept there is a problem, and then both people need to be willing to work it out. I know what you mean though, it really sucks to think that a relationship ended and you didn't even realize something was wrong.
Judging by your post, are you sure you're ready to really be friends yet? I think a friendship can definetly work, but you can't be trying to put things together or wondering why it didn't work out. If you're close you're going to have to deal with him being with other girls and maybe even him talking about them to you. If you're not over this guy, don't put yourself in a situation that will just upset you. Get over the break up, then be friends (Not trying to be mean, I've just been through this myself and have had many friends who made the mistake of being friends with an ex too early

).