Nothing's wrong with you. If being diagnosed Bipolar wasn't enough, one month after breaking up with you he doesn't only have a new girlfriend, he's already engaged to her. So yes, your pain is natural and justified. The anger too.

I know it's hard, but you need to stop looking at their stuff. It's not healthy and will only harm and hurt you more, and while he's "trying" to understand you (as if his actions were very normal) he's also minding his own business with this new woman.
Like Leed said, I don't believe this new romance will last long either. He's Bipolar, and even if he wasn't, the fact that this new woman is engaged to him after only one month is also a red flag.

Take care of yourself now, take this chance to be selfish and mind only your business. Do what you need. It will be the hardest thing you'll ever have to do, and it will take time, but you need to let this one go. It will hurt like hell, but you need to go through this so you can come out even stronger.