What do I do? (Read description)

today my bf and I were eating dinner and talking and he says “when I was 17 I liked a 19 year old in one of my classes”.... so after letting that sink in I’m like HOLD UP. (I’m 17 and hes 18) we’ve been together over a year. The whole time he was 17 we were together! So I said that to him and he was like “what no” and I was like yeah. Then he was like “we were off and on and nothing happened cause I only had a crush on her and she had a bf.” I just feel so upset that he liked another girl while being w me. Also him and I were never off and on we were arguing a lot cause he was being an ass. I mean this happened along time ago and we r happy now but I still feel so hurt. AND to make matters worse we were having sex in his car and trying to do doggy in the backseat, my legs started to hurt cause it was uncomfortable so he got mad cause I messed up his rhythm and ruined the sex he said. His dick went soft and he yelled at me for moving (cause I was uncomfortable!) while doing doggy. Smh. Thoughts? I just love him sm, am I overreacting?

EDIT: I get what u all r saying :( it’s just so hard to leave him cause I’m so in love, he was my first everything and I can’t even imagine us not together. We’ve been together a year and 4 months and he’s always so chill, never raised his voice to me ever. Lately though he’s been having trouble getting hard and tonight his dick was super hard and went soft cause I moved so he was upset that he went soft and took it out on me. Also after snapping at me he felt bad and apologized and kept trying to cuddle/ kiss me. Idk what I will do but thx for commenting.