Most Helpful Guy
He is no longer the issue here. What you need to realize is that he is out of your life and thus no longer a concern. Yes, he perhaps made an incredibly stupid choice judging by what you wrote, he is none the less unimportant now.
What's left for you is to move on and forget him entirely. Don't even ponder on what he did since, again, it matter not.
Most Helpful Girl
Everyone has told you to move on which is easier said than done. In my opinion you need to prove that you're better than she is. Not only will this give you a confidence boost, it will also make your ex more curious and interested in you (if you want your ex back).
You need to avoid him like the plague for a month, ignore all attempts (if he tries to contact you), work on improving yourself. Do the things that make you happy. Learn new things.
Maybe at the end of the month you'll realise that you don't need someone like this in your life. You're the one who is making yourself feel like this. Take control and be happy.