<human>: This girl and I have been dating for 14 months. We're very open and honest with each other. I know her fb password, and I just like reading her messages. Nothing too protective, I honestly just like seeing how she talks to people.
First, some background. Her last relationship ended when she went to get cabin and met another guy who she totally fell for, which made her realize she didn't like the bf anymore. Basically, she cheated on him. She's past it and I wasn't too concerned with it. She's told me a while ago that they made out, and "nothing more".
Tonight, I was reading her messages with a friend of hers. They went back over a year, up until about a month after the cheating. In the messages, she told the friend that she snuck out whilst at her cabin and gave this guy a handjob. When I read that, I couldn't believe it. I feel sick just thinking about it. I understand why she wouldn't tell me that, but she's made the point many times that all they did was kiss. 

I just don't know what to do. She'd be upset to know that I was snooping in a way, but I know I have to do something, even if it's just to post on reddit. 

I have a feeling this'll get long...well, I guess this mostly bothers me because I know she still has a soft spot for this guy, even though they can only ever see each other once a year. I was terrified last summer that she would repeat the cheating, just out of my insecurity. And the idea that she cheated on a guy is bad enough to me, let alone that she gave a guy and handjob while she still had a boyfriend at age 16. 

There's just a lot of insecurity welling up from me about her past and I don't know how to deal with it.

Can you condense the text into a few key points?
<bot>: My girlfriend told me she only kissed this guy she cheated on her ex with, but I just found out that she went further and it's really bothering me. What should I do?