Trigger Warning sexual assault: Need Help with overcoming sexual assault

TW: sexual abuseI’m writing this here hoping for some advice on how to move forward in my (first!) relationship despite some sexual hangups that are a result of my being assaulted back in grade 9. (I’m 19 right now)I’m a guy and when I was 14/15 I was sexually assaulted/abused by an upperclassman at my boarding school. Although only one guy did the actual stuff, there were a group of people who were also implicit, and it was under a banner of hazing when really I think it was because a combination of both I was one of the few people there on scholarship as well as probably some power thing.I was pinned down and had his finger and other foreign objects shoved into me, and despite all my yelling, nothing happened. It hurt a lot, and struggling was only making it worse, so I went limp and resigned to it until they left and I tried to hold back my tears unsuccessfully.I harboured a lot of anger for years, but it almost destroyed me so I have moved past that and now forgive the guys who actively participated and let this happen to me. However, in my first relationship, it’s causing some problems on my end.My boyfriend is 99% a top, we’ve been in a relationship for nearly six months and only a couple weeks ago did I tell him of this. Not always, but often, when we get sexual and he assumes his normal role as the top, I shut down, and only once have I experienced a full flashback that caused a lot of mental pain that was really confusing and scary for him. Obviously he isn’t only in this relationship for the sex (otherwise he would have left months ago) but I trust him 1000% and want to actually be able to make him happy sexually but I have a lot of fear/weird relationship with sex as a result of what happened to me in grade 9.Since telling him about this, he’s been very very careful. Before me telling him this he would check in (I think he knew something was up) but now every sound I make he checks in and honestly I love him so so so much, because he makes me feel so safe. I want to be able to move past this, and am wondering what you think I should do to make it possible for me to move past this?BUT: I have zero interest in going to the police, and I will not reach a conclusion to this by a legal battle. This is something I need emotional mastery over.