Most Helpful Guy
It's a shame young men don't learn relationship skills from their mothers like they learn things from their fathers. I think Marty is right when he says he probably wanted to pursue someone else but now that he's done that, he's remembering how good it was with you.
His rough breakup with you was out of ignorance in how to handle a mature relationship, I've seen so many young girls and guys treat the other like crap until they finally build up the nerve to admit what their real agenda is. You cut it short by asking the question that he was afraid to say - that speaks well of your maturity - you could see what he was striving for.
As far as getting back with this guy, I think he probably has a few more years to go before he is fit to be in a serious relationship. He was descent enough to at least end things with you before he started with her (I hope) but he still has a lot to learn about caring for another person. Perhaps maintain a friendship with him - not an FWB - and perhaps after a couple of years he might grow up a bit. I would however recommend you date around quite a bit in the interim. Go on lots of dates with a lot of guys, never date the same guy more than once consecutively, and get a feel for what you really like. Perhaps your old Boyfriend will look better or maybe you'll see that he really wasn't what you needed after all.
You sound like a really sweet girl and I wish you the best of luck.