<human>: Give me the gist of the text in a nutshell

Hi guys, first post here so apologies for anything that is a bit messy/isn't traditional etiquette. I'm a 21-year old guy in university.

The problem/question: I've been with a girl for about 10 months, and it's been more than friends to both of us (we've been on dates, sex, sleep together, spend more time together than just friends would and have a deeper emotional connection, etc.), but we never explicitly outlined us being in a clear boyfriend/girlfriend relationship, but we have mentioned that if either of us were to be with somebody else (dates or sexually), it would definitely hurt the other. I have let her know that if she was with another guy I'd be hurt, but I never tried to stop her from being with other guys if she wants. In recent weeks, things have gotten a bit stale which is natural, and I've been growing slightly suspicious, but I thought it was just me being stupidly paranoid (an insecure trait I've yet to fully overcome). Yesterday, she was out, and I made the mistake of reading her journal that was on the floor (I know I violated her privacy big time). In short, I learned that she had sex with another guy a few weeks ago, and has been getting physical with a few other guys as well. I told her we need a break, but also fessed up to reading her journal. We're both upset, but she said she doesn't want to see me go. 

I'm pretty lost as to how I should handle this and what I should've done before. She mentioned that if I was suspicious I should've flat out asked her about cheating and she would've told the truth. I should've just asked straight up, but I'm doubtful that she would've told the honest truth. I also feel like her hooking up with other guys is a reflection on me not being a good friend/boyfriend (in-between). I think part of me wants to stay, but also I'm just afraid that she'd do it again. What's your take?
<bot>: two college students, not bf/gf explicitly, but pretty close. She had sex with other guy(s). What should I do?