Having a breakdown. What went wrong here?

To be honest, I wouldn't believe a word he says. He is clearly upset that you had found another guy and started dating, whether it worked out or not is irrelevant. He obviously had some stronger feelings for you, otherwise he'd never have asked you if you were falling in love with him in the first place, but once he found out about this other guy, he was hurt, upset, angry, pissed off, and just trying to make you feel bad for not choosing him. I'm willing to bet he still has strong feelings for you, but is pushing you away with his insults and what not because he is afraid of being rejected, again, by you. Jealousy can bring out a lot of emotions and behaviors. Try not to stress so much over it and speak to him logically and rationally about it. Nothing will change or get better unless you can communicate what you are feeling to him and once you can do that, he is more likely to tell you what's really going on with him. Good luck!