OP: I think this is just normal. If it was a long relationship, it was probably a good one (though not necessarily), and right now you are just reminded that you used to have it, but now you don't. Or more importantly, it is now someone elses, and someone else who you feel like could be you.

Just let it pass, you know you can find other people too. And even if getting back together with him is the best possible outcome, you probably want things to cool down first.