Should I stay or leave?

Me (18F) and my boyfriend (18M) have been together for a little over 3 years now. Yes since high school. We’ve had some ups same downs. I mean that’s why I’m making this post lol. I just want genuine advice on this current conundrum.

This is my first real relationship so I don’t know much about what should be expected. Anyways. Me and my bf went on a break after dating for 2 years. I heard 2 years is the hardest and they were right. I can’t fully remember why I made that choice but I think it was just I didn’t feel respected anymore.
We didn’t respect the break that well. Me and my bf were still very much close and couldn’t bear doing no contact. We were even still intimate. There was a point where I realized we probably shouldn’t be. So I communicated that with him but made it clear I just needed space but I still love him and I will come back.

Keep this in mind. This break probably only lasted around 3 months. Short duration. This happened in the middle of our junior year. During the beginning of the year.
My bf had became “friends” with this girl in his class. She was a freshman (14F). She was pretty don’t get me wrong. That definitely made her feel like a threat. I get rubbed the wrong way abt making new girl friends. Maybe I’m just too jealous. Anyways. During this time I expressed my insecurities and he seemed understanding at the time. For the sake of this I’ll call her Emma. The friendship between my bf and Emma didn’t really seem to stop.

Flash forward back to our break. I’m walking in the halls and I see my bf and Emma walking with each other. Ofc my heart sank. Yes, I was trying to “move on” as well but not really. It hurt that it had to be a girl I was already insecure abt.
I saw my bf in class later that day and had asked him about it bc I’m nosey and was jealous. He stated yes he was trying to get to know her but didn’t want to peruse anything romantic. Something in me did not believe it for a second.

Okay, now me and my bf are back together. When getting back together I told him everything I did with whoever and so did he. I still had concerns abt the whole truth with Emma but I chose to believe my bf bc I trust him.

Flash forward to now. I randomly thought about Emma. Not too long ago, my bf was being threatened by Emma’s man. I had asked my bf why and he had told me he had no idea. I’m sure you can assume he knows exactly why. Forgot to mention, Emma had been with her bf the time of me and my bf’s break.

Anyways, thinking about Emma, I thought to ask my bf’s friend, “how much do you know abt my bf and Emma” he said, “everything” I was confused bc I thought nothing would’ve happened between them so how could there be anything to tell. In my mind, I knew deep down there was more going on between them. So I asked for more info. I’m thinking the worst, in my mind this is nudes or whatever. Nope. I just get the message “the fucked”, back.

My heart sank. My bf had kept this info about him and Emma from me for a whole year. How could he keep a secret like that. So that means Emma cheated on her bf for mine. Yes technically we weren’t together, but I was still sexually active with him around the same time they had sex. I was so heartbroken to have gotten that message. This was last week btw.

My first instinct was to call my bf and break up with him. I still can’t comprehend it honestly. My bf confessed to it so I knew it was true and I made him tell me details.
He told me, yes they did it raw, he only did it for like 2 seconds before realizing he fucked up, and he didn’t tell me in fear I wouldn’t wanna get back with him.
Guys I’m very conflicted. I’m trying to think of it as if I were to get with someone who already has a body so it doesn’t seem that bad. Basically convincing myself he’s someone new.

But this situation hurts the same way I think getting cheated on feels like. He tells me he was acting out of lust and he was just depressed bc of me breaking up and the sex meant nothing.
I mean even if that’s true he still went all the way with a girl in less than two weeks. What’s your opinion if anyone even read this far😭 I’m pretty unsure of what’s right and wrong in relationships. Should I just forgive and move on bc we weren’t together? Or is it something deeper?

TLDR: basically me (18F) and my bf (18M) had a break about 2 years into our relationship. During this break we had still been romantic with each other. We tried to move on with other people. I didn’t really talk to anyone new. Found out he had sex with someone else during our break and didn’t tell me for a whole year. I found out from his best friend. Is it valid bc we weren’t together even tho we were still intimate during the same time?