Cheated On

Me and my boyfriend have only been dating for a month but I can tell he is the one but here's the story: So we were just chilling in his room and so I asked him if I could look through his phone and he said yes. So I went and I opened his photos and there was three pics of girls asses and one selfie from M and one of the pictures said "what do you want my friend" so that made me think that he snapchatted her first but anyways so I asked him and was like who are these girls and he told me and I was like and they're from snapchat? And he said yeah and I was like and they were sent to you? And he said yeah and then I was like and you screenshotted them? And he said yeah. So I just got all quiet and upset. And then His friend came over and he was there for awhile so yeah but then when he left my boyfriend was telling me to like talk to him and so I was just asking like why he screenshotted them and who they were and he told me and then I looked through his phone again but he already deleted the pictures and so I went into his snap and he had a streak with them all but two of them were only like 17 and 15 days so he started them while me and him were together. And so I was asking more questions and apparently he didn't ask for them and he didn't respond to them. But he screenshotted them all and saved them. 2 were from a whole week ago and 1 was from just the other day. And so I asked like if he screenshotted them to just like show his friends? And he said no and that he didn't know why and it was just a in the moment thing. And then he went on to say shit like I love you and I'm so sorry and I'll do anything to fix this and I ruined this and you know all that. And idk it's just like if I have never found them would he have kept them? Would he have gone farther or done it more? Like idk I've done this before too so like I understand what happened but I was 15. He's nineteen. He should know better and he just keeps saying that like he had to fuck to learn from it and everything. But he should've known in the first place. And all these girls are single and go to his school. And idk what to do or what to believe like I know how much he loves me or at least I thought I did but all the trust is gone and I don't want to break up with him but I can't just get past this. It's always gonna be in the back of my mind. I just don't know what to do. When I did this I was in a year long relationship and I was broken up with so I feel like breaking up with him is what I should do. And I know some people may say this isn't cheating which I half agree but I can't have this in my life. We are adults. I'm going to college. I don't need this immature bullshit. Help. Do I break up with him or no.