I don’t know what to do😫

My boyfriend and I have been dating for almost 4 years now and he told me that when we broke up months ago he almost slept with someone he saw as “family” as he has known her longer then he’s known me because he was best friends with her brother. But he didn’t tell me in a way where it was calm and stuff he broke up with me so I walked to his house to get my jumpers back and a phone I let him borrow as he didn’t have one at the time ( he completely smashed the phone I lent him ) we started arguing and he said oh yeah and I almost slept with someone wanna know who it was Darby he yelled it at me from across the road and walked off I then dropped to the floor crying just feeling my heart sink and it became harder to breath and then started sucking up and shit ( we are still together because I want to believe he will change) but to this very day when I ask what happened between them he refuses to tell me he says he doesn’t want to talk about it and that it’s making him uncomfortable even tho he is the reason shit happened that night because he was alone in his room with her and said to her I don’t know how to kiss can you show me and apparently she was leaning over the top of him with no shirt on but he says nothing happened and I know for a fact he is lying whenever I bring it up he accuses me of cheating and doing shot when I never have. Then on New Years at about 12:03 am a random number texted him saying “Alg” and there was no previous conversation before that like he had deleted it and he got defensive immediately and we had a massive fight. I need advice! Oh and he is 19 and the girl was 16 at the time this shit happened. Even thought me and her were friends and she did this shit behind my back and couldn’t even have the decency to tell me.