Most Helpful Guy
The thing that sucks about relationships anymore is that we get in too deep and too fast. In times gone by, people would just date around until they were pretty certain they had found the right one then they start going steady, get engaged, and then start sleeping together. Anymore we tend to be attracted to someone and by the third date we're intimate with them. Once you've been that close to someone, heartbreak is almost for certain when there is a breakup.
The big problem here is that you got too close to someone who wasn't ready to commit for the long term, you got your heart bound up with him much more than he did with you. He decides he's not ready to commit to you and wants to pursue other opportunities and says "I'm not happy" and cuts it off like that. You were probably thinking about children and a house in the suburbs, he's thinking about chances he is missing.
The truth is that you'll never show him he made the wrong decision, he'll have to find that out for himself. After he's been around the block a few times and sees that nobody loved him as much as you did, he might figure it out, but then again maybe he won't. But you don't need to wait around for him to figure it out.
What you need to do is to focus on you for a while. Focus on your work or school, focus on your family, go to the gym and get toned up, do things to keep yourself from sitting around watching TV and feeling bad for yourself. After a couple of weeks, start dating. Don't let yourself get serious, date a different guy every week - don't let anyone get serious. I think that after a few months you'll find that you feel a lot better. Learn from what went wrong and don't let yourself get intimate with anyone that isn't seeking the same things you are, make them prove how serious they are before you let them get that close to you.
Your story isn't anything new, I've seen it a thousand times. My suggestion isn't anything new either, but it works - trust me. Be strong and you'll learn from this and grow stronger. My heart is with you, go out there and do it right this time.