My girlfriend has been flirting with another guy, kissed him, and is very likely going to have sex with him tomorrow on the first day of her vactaion, while I'm at work. She doesn't know I know all this, because I found out by accident. I saw photos of her and him on her phone while sending some pics she took of us to my own phone, and the date on the pics were a night she told me she was somewhere else. Out of confusion and desperation, I did something I thought I would never do and checked her messages while she was asleep... and found all of this out. Since then, I've been racking my brain trying to figure out why she would do this... she seems incredibly happy with us, spends tons of time with me even when she doesnt have to, and invited me to her parent's home today for dinner, seeming distraught about the fact that we wouldn't be seeing each other for a week. I've even talked to her about us, our weaknesses, what i could do better as a boyfriend, and about cheating and she tells me I'm the best bf she's ever had, and she would never cheat... yet she's made out with this guy twice already... once he came to her workplace and they went out for lunch, and they kissed... he's even felt her up... and she's actively kept the fact that she has a boyfriend a secret from him. I cannot wrap my mimd around this and at this point I'm just waiting for her to sleep with him so I can finally confort her... I've been archiving the messages so i hsve evidence... but i feel so sleezy about reading her messages, and conflicted about the whole situation... i just dont know what to do. I want to confont her now... but in some morbit way... i kind of want to see if she would actually do it. This woman who devotes so much of her life and love to me... or so I thought...

TL;DR: girlfiend is cheating on me and I'm lost and confused.