I'm sorry you're hurting so much. I have been in a very similar position with my husband while we were dating.

I hope this isn't offensive (I'm sure I would have taken it as such), but, why does it matter what he did during your break? Were there rules in place that this was just a break, not a break-up, and that neither of you would start seeing other people? Or was this a break-up, with no intention of getting back together? There can be hope of getting back together, but if the intent of the break up was to break up, then you need to try to stop focusing on what he did. What he did during that time was not a reflection on you. For all you know, that's how he copes with a break up.

Can you pinpoint exactly what is upsetting you?