Self esteem issues

I’ve been with my boyfriend for 8 months and I still can’t stand his ex. It’s funny cuz shes the only girl that I get upset about that he talks to. I mean they don’t talk 24/7 an he doesn’t try and break his neck trying to talk to her but they exchange hellos and they laugh over things and stuff and it makes me sick. I never liked his ex even before we started dating. I’ve asked him why he talks to her after the crap she put him through when they were together. (she cheated on him). He said he just let it go and got over it cuz he said it ruins good things and by that in guessing he meant saving her as a friend. She’s said she doesn’t have feelings for him anymore and I believe her a little it’s just a part of me that doesn’t. I’m not sure if its my non trust in her or in him but I think it’s both. He knows I have a hard time trusting him and he assures me that he’ll never cheat on me cuz my past with guys has been awful. All my past serious relationships the guys have talke crap about me behind my back, cheated on me with their ex, and his plain dumped me cuz I wouldn’t have sex with him whenever he wanted. So I’m pretty sure that’s the root of my trust issues but I can’t help it. I want to stop this because I don’t want this to put any more strain on our relationship. He has trust issues too he just doesn’t vent it as much as I do. Every time i see him and his ex talk it makes me automatically angry, it’s like someone flipped a switch and then in mad at him for a while then I get over it. I don’t care that they talk the main reason is the thought of them two together an knowing that they use to be together makes me edgy. I’m just scared of losing him and being hurt. I’ve already told him all of.l this hoping it would make me feel better but it doesn’t. I’ll be fine for a while then the thoughts just come back and I’m sad again. What’s wrong with me?? I don’t know what to do. I feel like its making me crazy.