Can't stop this obsession with my boyfriend's ex?

i have these mixed conditions?
i sometimes feel that me and my boyfriend's ex are alike, which makes me want to throw up and makes me hate myself. and somewhat i feel disgusted.
at other times, i feel she's way better, prettier and all. and that makes my self-confidence decrease.
i start to think poorly of myself, and self-hatred strikes again.

sometimes i notice a resemblance in her attitude and my boyfriend's. since they had been together for a while. and that also disgusts me. i mean i always end up feeling sick.

i want to stop comparing and i want to stop stalking her, i tried am awful at that.
i need someone to slap me whenever i do and fuck i'm just too insecure these days.

i don't want to talk to my boyfriend with that because like i really am not comfortable of sharing those ideas with him. but i'm really anxious all the time with that.
help?