Why am I having a hard time letting go?

It’s been 8 months since me and my baby’s father split. I have so much anger & hurt I have for him. He’s already slept around with a few people & I haven’t. I haven’t felt ready nor met anyone I wanna share my body with. Anyways, I’m annoyed I still care what he’s doing. I know I’m getting better bc it doesn’t hurt as much as it did but I still get major anxiety on a daily basis when I think about who he’s with or what he’s doing I’ll actually get knots in my stomach and my heart will race. I’m tired of it. I don’t want him to have this hold over my life anymore ☹️ I don’t even want him back