AITAH for something i did in the summer?

So i (f18) and my bf (m18) have been together for almost two years. about a year into our relationship my boyfriend out of nowhere started telling me how things weren’t working out. he said our arguments were too much for him and he couldn’t deal with it anymore. I understood that however i was devastated. He was really wish-washy for about 2 months as he would say things like how he’s happier just spending time with his friends or he wanted to go no contact so he could let go of me.

Obviously i was extremely upset and was honestly just clinging onto any aspect of him i could as he’s my first everything and i am his. about a month into me kind of begging for his attention in anyway possible he continued to come over and do things w/ me but then leave right after, tell me we needed to stop talking and go hangout with his friends/go to a party/ hangout w friends and other female friends. this really hurt as this wasn’t how he would act during the relationship and it was new to me. I was gutted and after that two months, i decided it was time to stop being pathetic and start focusing on myself. I got the closure i thought i needed after he confirmed he just wanted to have a good “senior summer” and i realized i deserved a good summer too.

I agreed to no contact and as i said, focused on myself. After a few weeks i posted a collage of pictures of myself on my instagram acc. and he started talking more frequently to me which i believe is because he didn’t like the attention it got from other people (i speculate this because of things he’s said, he’s never flat out said this). For about 3 months i denied and said that im healing and im happy with where im at. during those three months, i began interacting with what was pretty much the first boy i had been personal with ever besides my ex boyfriend. We had as i said, personal conversations and after hanging out a few times we got high and had intercourse. I immediately was sick the next day after i fully processed what had happened and told the boy that i wasn’t ready for anything intimate and i wouldn’t be for a while. the boy and i stopped talking as he was respectful about the situation and my choice to stop.

My ex boyfriend and i started to rekindle about a month after, this was after he had showed up/driven past my house multiple times which wasn’t okay but he’s grown from that. it was just something that we both had to go through and i think is important to why i wasn’t talking to him. But fast forward to now, my boyfriend and i have gotten back together.

we got back together around september and i immediately told him about the boy from the summer before completely in a relationship. he absolutely did not take it well and i completely understand that. but since that, which i have been very open with him about, he continues to bring it up and i will ALWAYS reassure him and help him get over this. But is there any ways i can help besides reassurance? sometimes it’s used against me in disagreements or we’ll be having a good time and it completely ruins his mood as he says his whole time of like the start of our relationship to now he only thought it would ever be me that he had done things with and vise versa. I just am lost and it continues to be brought up and i feel horrible. like absolutely terrible and i don’t know how to help him. please let me know if im not seeing something as i should or if i need to change my perspective. thank you in advance!

also, yes, i know we’re young but please don’t use that as leverage to excuse it as typical teen behavior (ive seen it happen on other posts) i just need a little help with the situation