SUBREDDIT: r/relationships
TITLE: Me [20 M] with my Girlfriend [20F] of 8 months; found out she cheated early on through reading her FB messages
POST: My girlfriend, who is the first girlfriend I've ever had, left her facebook logged into my laptop. Curiosity got the best of me, and I decided to check messages she sent to her best friend.
I just wanted to go back to messages at the start of our relationship/ when we first met to see what she initially thought of me. While I was scrolling up, I got to around a month after we started going out and I got to this.

*"I made out with (Not Me) last night...Like I knew I would shitttttt I knew it was gonna happen and I kept tryna say no but I couldn't do it! he's pretty good looking and soo so nice:(..I know!  I feel so bad but I don't know if anything else will happen with (Not Me)*"

This was around a month into us going out. Im pretty upset about it. In her defence, I know it was a time when we weren't sure about were we stood since we had just become long distance but we were still in a relationship.

The part that really gets me is the "I don't think any thing else will happen" part. Like she was already lining someone up in case we broke up. 

I dont know the guy since he lives where she studies and she'll know her best friend didnt tell me so the only way I can talk to her about it is if I tell her I read her messages, which I feel really bad about. What should I do?
TL;DR: 
Found out my Gf cheated by reading her messages, feel awful for invading her privacy and dont know if I should tell her.