<human>: Can you give me the gist of the text in a nutshell?

My 22 year old girlfriend and I [27M] have been together for 2 1/2 years, with the last few months being a little rocky. In the past couple of months she has spent a lot of time with a friend of hers [24M] having drinks and spending time alone. I try to be understanding and let her do what she likes but I've always been jealous/uncomfortable with the situation.

The jealousy in me eventually reached a point where I started reading her text messages and iMessages on her iPad, something I've always refrained from doing in the past. I knew she talked to a few people regularly so I read the messages and realized she had sex with her friend. I also read a conversation saying she regrets the action and should never have done it.

After finding this out, I became infuriated and incredibly upset. I want to talk about it but we're in such a good place emotionally right now. The only reason I haven't mentioned it to her or confronted her on it or knocked out the guy who had sex with her is because I only know this information from reading the messages and otherwise wouldn't know. My question then becomes what to do. Should I confront her about it or wait until she talks to me about it?
<bot>: My girlfriend cheated on me, she doesn't know I know, I only know because I read her texts, what should I do?