My partner has been hanging out a lot with his ex-girlfriend and I'm hating it.
Why don't you want him to hang out with his ex?
He is so much in love with her. It hurts my feelings
I get what you're saying, and having those strong feelings definitely makes it a little bit odd. I'd recommend you telling him this and hopefully he understands and will stop hanging out with her.
I'm glad you're seeing my side of things, I think I will try to talk to him about it.
Communications going to be the best thing for you in the situation, just make sure you approach it the right way. Hopefully your boyfriend sees your side of things and you both can work on getting this resolved.