Is it normal that i can't seem to get over my ex?

Ok...so I was with this guy for 3 years. We were childhood sweethearts, he was my first everything blah blah. And I know many people say this about their relationship, but ours was as close to perfect as one gets, I think. He was my best friend too... We had always said that (freakish as it may sound) we wanted to get married etc...but that we'd take a few years break to see were we meant for eachother first! So then in August it seemed the right time to do so as I was starting my first year in college and we'd be doing the long distance thing if we were to stay together. But in the few days after we broke up, I ended up being with one of my best friends and started something up with him in secret(mustv been rebound!) Then about a week after me and my ex ended up getting back together but I was still in contact with the best friend guy! Anyway- so we only got back together briefly cause he found out i'd been with the best friend and things went terrible! So then a month later I find out my ex is dating someone else...that was 5/6 months ago and I'm still in bits. I've met him since and asked for an explanation but he can justify all his behaviour based on the fact that I rebounded briefly with my friend! He still wants to do the marraige thing in years to come and we still get on SO well when we see eachother...but he's with someone else. And I can't move on. And I can't blame him cause I ruined it. But he was left hurting for a month...I'm still hurting 7 months later. I don't know how to move on cause there was no proper reason for our break up...yet there's no going back? Don't know if i've explained it well but yeah...Just don't know how to get over it...?