This is about to be long because it requires lots of context. Sorry in advance for any mistakes, English is not my first language.
My boyfriend and I have been together for almost 4 years in total and we have known each other since the first year of college. I consider him to be my soulmate; not only is he my boyfriend, we have a great relationship based on a deep friendship with mutual respect and admiration. I honestly think of him as my best friend.
We faced some issues (I was going through a crisis) and as a result we broke up. During the breakup, he turned to other people, including his girl best friend whom he got closer with. They live 5 minutes away from one another and they have known each other since they were kids so it was only natural. Also, since we had the same group of friends and he wanted space from me, he started hanging out with them more.
During the breakup, I started having personal issues with his new group of friends because they mocked me and other friends of mine, so I decided to cut them out and go on about my business.
A year after the breakup we had found out way back to each other and decided to get back together after sitting down and having a heart-to-heart conversation. It had been a tough year for both of us breakup aside: issues concerning both our groups of friends, his mum getting a cancer diagnosis (she's doing great now) and my last year of college being the most challenging one yet. We found confort in one another and decided it would be the best for us.
Moving on to his girl best friend. We got along while he and I were together and during the breakup she continued liking my stories. She became part of my boyfriend's new friend group that he met while we were not together and in particular she became very close with the girls who were mean to me and my friends. I talked to my boyfriend about how it made me insecure and explained what had happened between me and those girls and he didn't believe me at first because, in his words, "he knew them better and they weren't like that".
That was the start of one of the hardest periods of my life. It had been a year full of changes and I craved calmness, the one person I trusted would get me and comfort me didn't believe what I went through. The combination of all of this was too much for me and I developed insomnia for months (wouldn't wish it upon my worst enemy). Moreover, he and his girl best friend had picked up the habit of doing everything together and going out drinking every weekend. As a result of all of this, I lost trust in that girl.
That didn't stop there. When we were together, she would call him non stop asking where he was, if I was already with him, when I would get home so he could hang out with her… And also bombarding him with messages. It obviously made me upset and I would tell him and he would speak to her but nothing would change. I also learned that she asked him inappropiate questions once.
During all this, he tried supporting me the best he could and we even went to couples therapy, I was at my absolute limit with all the stress from college and the exhaustion of getting no sleep. However, our main point of arguments was his girl best friend. What was the most heartbreaking to me was that we usually shared our views on people but he would NOT GET what was so upsetting to me. It litterally took him months to see it from my perspective. Even his family was concerned about my feelings after she was at his house day after day (I didn't know about this until later).
He finally understood but it didn't stop. One day I had got out of an exam I studied really hard for and didn't go well. He was going to come for me and have some time one on one after my exam, but when I tried calling him he didn't respond to any of my calls or messages. I was furious and when he finally answered, he had been with her because they had an argument the day before and wanted to make up. That made me feel like I wasn't his priority and seriously thought about breaking up for good.
After I finished my exams, he would come see me and bring me snacks to cheer me up but the calls from his friend wouldn't stop. I want to add that everytime we argued about this, he would reassure me and do his best to support me but I felt that the only solution was to cut her out but obviously I couldn't oblige him to do that.
When I thought the wrost had passed, my grandma fell ill and passed away after a month in the hospital. It broke me and my family and again, it was one of the hardest things I have been through. He was there to support me again. However, his girl best friend wanted to throw him a surprise farewell party because he found a job out of town. The day of the party was the same day my grandma died so obviously I didn't go, nor did I plan to. Merely three hours before her passing I recieved a message asking if I was going to come. She and my boyfriend's family were aware of what we were going through, but not even once did I recieve a check-up message on my grandma from her.
The final straw after that was two weeks later when my boyfriend presented his thesis. I couldn't attend because I was out of town, but he didn't tell anyone except his girl best friend who then went to celebrate with him and his family. We had a huge fight after that and he understood that he had to distance himself from her if we were going to stay together and he finally kept his word.
Three months later, we're still together and thriving, he still sees her once a month or so, and the blowup messages and calls have stopped. He's distanced himself from his other friend group that turned out to be toxic. Everything seems back to normal. However, when I see her messages pop up on his phone when he wants to show me something I am reminded of everything that's happened. I ask myself if I have forgiven something that would be a deal breaker for me if I wasn't going through such a hard time. I feel like I am not ready to let go but must if I don't want to keep resenting him for that. Most of all, it's not him who I don't trust but her and her influence on him. I've run out of ideas on how to cope and there's barely anything left to say between us about this. What should I do?
TL;DR: I can't forgive my boyfriend after he let his girl best friend get in the way of our relationship even though he's distanced himself from her.