I mean, unless you're really well adjusted, you're probably going to feel shitty for at least a bit when your ex finally moves on, even if you think you've moved past it. I knew ending my last relationship was the right thing to do, but I still felt blindsided when she told me a couple of months later that she was seeing this guy. I knew the relationship hadn't benefited either of us for a very long time, but I still cared very much for her, so knowing she was entering that intoxicating new relationship phase stung. But people move on at different paces. I feel I'm close to being ready to meet someone new, but first I had to take some time for myself and also evaluate the relationship and my role in its demise.
Also, there's the issue hendrix mentioned. Getting back with her would be a mistake for both of us, but I'd be lying if I said I would reject one more roll in the hay. Despite everything, we had a pretty strong physical chemistry, so I don't know if I'd trust myself around her for extended periods of time.