So why do I still miss him?
Because you're human. And when we care about someone who's not in our lives anymore, we grieve the fact they're not there. You can miss someone while acknowledging their presence would bring you pain.
Why am I jealous of his new AGF?
Are you?
When my AXGF dumped me for another addict, I wasn't jealous at all. Nor did I feel bad for him. When someone shows you what they're all about, it's usually a good idea to pay attention.
If you know what your AXBF is all about, you surely know what the new girl is in for.
She may not know, but that's not your problem. That's hers. So what's there to be jealous of?