I’m letting my retroactive jealousy ruin my relationship

My bf has an ex fwb from before we met who I’m struggling over. I think about her and them constantly. I hate the idea that they were together, even before we got together. I hate that he slept with her and found her attractive. I hate that he had to text her that if they hung out they couldn’t sleep together because he just started seeing someone new (me) and didn’t know where it was headed but didn’t want to sleep with anyone else, I hate that she’s texted him upset that they haven’t talked in a long time and feels like their longtime friend is wasted.

I hate that I think about her all the time and have this obsessive worry that she’s pining for him. I hate how much I’m letting it ruin my relationship.

My bf had to delete Instagram because I had issues with what he was seeing. We had also argued multiple times that I didn’t like that they were still friends on ig and asked him to either mute her or delete her but he outright refused because it “wouldn’t be fair to her” because she did “nothing wrong”. We can’t even talk about her without arguing. I have no idea how to get over this immense anger, jealousy, and obsessive thinking whenever she comes up.

I’m in therapy, I journal, I’m doing the very best I can to control my impulses and behaviors but I STILL worry that every time he gets a text, it’s from her and that he’s secretly hiding it from me so as not to upset me. I know he wouldn’t do this, he’s not that type of guy. He’s wonderful and has been so patient but his patience is already gone with this issue, it’s been a year so I don’t blame him. We discovered I might have OCD together so that’s given him some patience but he still gets upset and tries very hard to be as patient as he can.

I’ve recently turned over a new leaf in my own head about a different friend of his but because of that, now my brain is only focusing on this one girl. I recognize my thoughts are lying to me, I’m sure she’s not in love with my bf and wants him back, I’m sure she’s not this awful person my brain made her out to be but I still can’t relinquish this obsessive desire to have control over their friendship. I want to have a serious talk about him getting Instagram back because we enjoyed sharing memes together and he liked being in contact with old hs and college friends but I have this worry about them talking again and him looking at her pictures. I know trying to control him is unhealthy but I worry that by letting go, I’ll regret it or some bs. My therapist said to avoid triggers as a from to get over OCD, I figured ig would be perfect for that but it seems like too much at once. Even telling my bf that I want to have a serious conversation about getting ig back and how it could maybe help me and my triggers would be good for us but he has so much anxiety and trauma built up from all of our previous arguments.

I need tips and advice because I’ve wasted so much energy worrying about this girl who I’ve never met and lives far away enough that I’ll probably never run into her. I value my relationship so much, my bf is wonderful and loving and I would absolutely hate myself if I were the culprit for our relationship ending.