How can I be happy for my ex?

We broke up in June, I should say our relationships were kind of toxic for both of us, so I made a decision and he finally agreed with me. Anyways, I just went to his Facebook and saw a picture of him and his new girlfriend. They smile and laugh, hug each other, I wanna be truly happy for them, but all I have is jealousy, pain and negative thoughts. I even went to his new gf page and while going through her pictures couldn't stop thinking how much I'm better than her. That's so horrible :(

The strange part is - I don't want to be with him. If he calls right now and asks if I wanna get back together, I'd absolutely say no.

What's that? How can I get over this crap, stop acting so insecure and start being happy for him?