Boyfriend troubles

Hi everyone, just came on here to get some help regarding my situation. My boyfriend and I haven’t been on good terms lately and I’m just looking for some advice.

So a little back story, we’ve been together 4 years everything was good until we moved to a different city (his hometown). He has been trying to make friends and he has but that’s the issue...his new friend is a girl 7 years younger than him. I was all cool with it until I left him at a party because I worked the next day. He didn’t come home til 6 in the morning and when he did he told me to put his phone on the charger. Well when I did the lock screen came up and a bunch of messages from that girl who is now his “friend” it said a bunch of stuff about hoping he feels different in the morning and not to do anything he’d regret. Well this caused a fight because I have the impression he cheated on me and he tried explaining what happened but it all doesn’t make sense to me. Every time I try to talk to him about it he gets mad and blames everything on my anxiety, that I’m trying to start shit that isn’t an issue. But honestly I’m just trying to find out what happened that night...he says he was too drunk to remember and it’s killing me...we’ve been fighting ever since about little things that don’t matter and it’s getting so bad I fear we’ll lose our relationship. Also now he is saying he had a heart to heart with her about relationships and sex and he said he talked to her about this because he feels our sex life is “vanilla” in other words straight forward kinda boring. But it hasn’t been? So I’m confused and sad and just a mess, I don’t know what to do. I feel helpless and that whatever I do won’t help. How should I go about this ? I obviously want to stop thinking about it so that I never bring it up and upset him but I need to talk about it otherwise it will drive me crazy. Sorry for the long read, I hope you understand what I’m saying and can help. Any advice will help really, thanks for taking the time to read this ❤️