May be leaving you has more to do with how he sees himself rather than how he sees you. I am afraid this does not help with the feeling like shit part. But when the dust settles a bit, you might want to explore this angle. It might be his own sense of inadequacy that led him to leave you. It might have nothing to do with you.

I believe dating is a phase where two people are trying to decide if they are a good fit for each other. He simply found a better fit.

I'm betting there is absolutely nothing in the world wrong with you. I hope you don't let this experience color how you see yourself.

Dig down deep inside and ask yourself this: Did what you have seem like true love? Were there rockets, fireworks and that knowing deep down in your soul that he was your one?

I'm betting it was nice, comfortable, intimate and loving but not the end all, be all of romantic entanglements for you. If you shared that particular connection, you would have known it. Instead of being anxious and unfocused there would be a gut wrenching sense of betrayal that ran so deep, your soul would feel fractured.

He should not have led you believe the relationship was serious and exclusive if he was still seeing other people. Did the two of you ever speak directly to that issue our was it simply implied as the relationship developed? If it was implied, I'm not certain I'd even been upset.

Just so you know, I was once were you were and it really twisted me up for a minute. I was young and it really felt like she had something that I was fundamentally lacking. I went through a lot of feeling like crap and soul searching to figure out why I didn't measure up, only to discover that he dropped her a few months after that for yet another woman. Then I had to laugh, because it was clear that it had nothing to do with us. He was just one of those grass is greener guys. I think reasons don't really matter. Moving on as quickly as possible to minimize the pain does matter.