I (22m) went through my GF (22F) phone

I messed up

Ok I know snooping is bad and I feel guilty for doing this but I need to get this off my chest. My Gf (22) and me (22) have been dating for 3 months now and things have been wonderful, she is literally to greatest woman alive and I love her to death. We met on a dating app and were just friends for 2 months before we became official and started our committed relationship. But the thing is during those 2 months nothing was different. I treated her like my girlfriend and she treated me like her boyfriend. This is where I made a mistake. So recently my girlfriend has just gotten a brand new phone and kept her old phone as a backup. She hadn’t cleared her stuff off of it yet so it still had all her old texts and photos. She was at work and I was at her place cleaning up. I saw her old phone and I don’t know what came over me but I started going through it. This is where I came across a name I remember seeing when we first met. I went through the messages and they were very explicit. I was shocked. I looked at the dates and it was 2 weeks after we had first met. I know this isn’t technically cheating because we were just friends and she was free to see other people but it hurts because back then she told me she wasn’t seeing anybody else. I know I’m in the wrong because I shouldn’t have been snooping and I got what I deserved and I just feel so guilty for knowing something that I wasn’t suppose to know. How do I confess to her that I know she lied?

Edit: based on the messages they did meet at least twice and they did some explicit flirting about they’re time together. Honestly I’m not upset about the whole old fling thing, I’m more upset with myself and how I betrayed her and now have this secret that she deserves to know about but I’m just too scared of the outcome.