Advice please ?(A little long)

I don’t really know how to start this off considering this is my first post. My boyfriend and I have been together for one year and I’m only 16, I’m glad to have him by my side but one thing always comes to my head. A while back he did something that truly hurt me and I can’t seem to forget it no matter how hard I try. My boyfriend had been texting this girl and when I asked Who it was he said it was a cousin. I believed him of course because he hadn’t done anything to me that would make me not believe or trust him. Time went by and as a normal relationship what gotten into an argument and during the argument he was texting this girl letting her know what was going on. The girl immediately responded with omg I don’t like her break up with her keep in mind that this girl lives 4 hours away and doesn’t know me or has never met me. We were still arguing when he stopped texting me so I logged into his Snapchat to find text messages with the girl and her saying to break up with me and saying terrible things about him and him saying terrible things back.... he said that he was going t9 break up with me and things like that I scrolled up to their older messages and found out he used to talk to this girl (flirt). For me to read those horrible things that he was saying hurt me I couldn’t sleep the whole night. I used to think girls who said they were up crying all night were being dramatic but I found myself doing it the night before. I woke in the morning he said that he would talk to me I went on to see that he had been commenting on her Instagram pictures and when I asked him he said it was his cousin that was doing it (his real cousin) I was so hurt I couldn’t eat my eyes burned I felt so empty something I had never felt. Just writing this story gets me upset.. but continuing I said alright lets talk he came over in his car late at night I went into the car and nothing but silence filled the air finally I began asking for an explanation but I didn’t get anything he was simply quiet it was like talking to a wall. More days passed of us trying to work things out and one day he broke up with me I was hurt and saddened but I understood I decided to go to a football game with my friend. I knew that he would be playing (ex) and he had texted me a few hours after our breakup asking to talk one more time. Of course I went at the end of the game he came up to me and asked me to go with him I followed him to the lockers and waited outside for him to finish getting dressed while I was waiting so many things were going through my head. I was scared nervous and trying not to cry. He finally came out and he stayed quiet and finally said that he loved me and wanted another chance that he didn’t know what he was thinking. I of course took him back every thing smoothed over. But his cousin the real one began disliking me because she was friends with the girl that was telling him to break up with me. It was around the fair time and she would constantly post pictures of the girl and would caption it “come get your girl(my boyfriends name)” which would upset me because she knew we were together. She took a picture of me and my boyfriend and the fair and captioned it “come get you man (girls name)” which upset me because I would think she would have respect at least that I was dating her cousin I confronted her about it and she said nothing I asked my boyfriend to tell her something and he did and she deleted the post she apologized to me and all and I also confronted the other girl through to text she proceeded by calling me dumb for staying with a guy that was talking about me and I said she was dumb for trying to steal him from me from 4 hours away. My point is it’s been sometime since this incident but it still hurts me I talk to my boyfriend about it and he tells me not to worry and I really want to trust him but whenever he goes with his cousin it scares me since she is friends with the other girl. I’m scared and it makes me feel terrible when i think about it but i just want to stop worrying has anyone been in this same situation and if you have please give me advice.... i just want to be able to trust him again and not feel so insecure around his cousin.

P.s. I’ve been really debating on posting this because I don’t feel like I’ll get a response but I’m hoping I will. I’m sorry if some parts don’t make sense it’s 2 in the morning right now 😂🤦🏽‍♀️