how to get rid of feeling that your SO is cheating on you? Pls help

my SO and I have been together for year and I love very much and i believe that he loves me as well. We’ve been through a lot together especially with me going to the hospital and all that and he was always there for me. I really do love him but around in August some things between us happened where he lied to me and broke my promise about not talking to this girl again that we were friends with (she manipulated me, threatened to kill herself if I don’t do something, etc). And I found out one time that he talked to her over about something which I moved on from it because it was about me lowkey. Its a very messy story lol but anyways, there was another girl who caused us a lot of drama that we promised not to talk to either (he also used to have a crush on her and used to have a crush on him until she trashed him,etc but they used to be good friends) and I found out a month ago that he was talking to her but he didn’t tell me. Of course I got angry and sad again. And then he told me yeah I talked to her but before that, I didn’t at all. I said ok I believe you. Then 3 weeks ago I found out that he had small convos with her in September, August, and October. He lied once again. We got into another argument, he apologized again and said he will change. Fast forward to last week, one of his girl-friends texted him “I’m having a bad day” and tried to call him before I met up with him and he did not tell me. Maybe I was being paranoid but we got into another argument and kept asking why I don’t trust him. And it was because of the past experiences and all that. Now I cannot stop having dreams of him cheating on me and any time the thought comes to my head I start to shut down. I really do love him and I do think he’s not intentionally doing this, he just wants friends and we all need friends since ever since I got out of the hospital it’s only been us and I do think he does not think when he does something so I don’t think its his fault but I can’t get out the feeling of him that he might be cheating on me or something for some reason.

I also have really bad trust issues and other very bad mental issues (why I was in the hospital) so him losing my trust kind of made it worse. I want to be able to trust him again and go back to normal from when it was when we started dating.

I’m sorry that this is long I really do need help tho.