Saw something I shouldn't have

A😌

Ladies I need some advice I was messing around on my bf's phone while he was downstairs talking to my dad. I was just messing around taking pictures on snapchat I swear (I've never felt like I needed to check his phone) and I saw this chick private messaged him. Well I opened their convo cause I didn't know who it was and Saw saved comments of his on her stories. Pictures of her in towels or with her cleavage all out and him complimenting her. Now some of these saved messages went back to 2016! We've been dating for 2 years! The most recent message was just a few weeks ago. I asked him about it and he took the phone from me and told me it was nothing. That's when I kinda lost it. He says the only things said between them was what I saw but Ik that's not true because I saw she private messaged him that day! The message just wasn't saved. I've never had issues with him about something like this. When I hear his voice I feel like he's telling me the truth but logically the story doesn't make a lot of sense. Even if there wasn't any other messages than what I saw.... am I really ok with him messaging other girls like that for months!? I don't know what I should do I love him and I want to forgive him but I'm really hurt and I don't want to push what I'm feeling aside just because I miss him.