How to trust again

I’m struggling with trusting and it’s been very apparently lately. My bf and I met when we were 14 and he had this other girl skip school with him and she went down on him while we were dating and it was a huge thing and we broke up until 10th grade (15 years old) and we’ve been together ever since. He hasn’t cheated since but for some reason I always wonder if he will again. We are 21 now and things have been fine but there’s always a “what if” that goes through my head. I hate it cause it’ll be at random times. He’ll go to a friends house and I’ll literally have a panic attack if I can’t go with him cause idk what he will be doing even though I have his location and is very close to his friends. what can I do? He told me that if I chose to stay with him I can’t continue not trusting him. Basically he told me either we move passed it or we don’t and break up. He doesn’t wanna hear about my trust issues anymore cause he thinks we should be over it by now and that he’s proved himself. I think I may have told him and myself I trust him but never actually healed from what happened years ago.. how do you truly begin to heal and trust? I love him and he’s a great man and I don’t want to lose him over something that happened in the 9th grade