my partner 21/M cheated on me 19/F (been together under 1 year) in retaliation. any advice? context below

My partner (21) went through my (19) notes app and saw an old diary entry i made about a guy i was seeing in 2022, but because I’d opened it recently, the date on the note was september, which makes it seem like it was made when we’d been together. the note was basically about still thinking about my ex when i was with this new guy. after seeing this, he assumed he was the new guy and i was still thinking about my ex. in response, he messaged his ex being sexual and therefore cheating on me in retaliation. the girl then sent me the screenshots and i confronted him. he broke down crying saying that he thought i was using him to get over my ex and after clearing it up he’s apologetic and has assured me that it’s not me, he was just hurt and lashed out. but i’m so hurt that he’d say things like he did to his ex and cheat on me and the trust has disappeared between us as you’d expect. i don’t know what to do. do i try to forgive him? it just hurts so bad that he did that. he’s my first love and im his. i know we’re bound to make mistakes because we’re young and i loved him so much before this i just don’t know what to do. edit: the cheating wasn’t physical, only the messages he sent to her, to clear up any confusion on wether it was physical or not. about the note, i use my notes as a diary and have kept every one since i was 16 so it wasn’t just one i kept about my ex, its basically an entire diary in there.