Did I make the right choice breaking up with my girlfriend?

I'm 22 and my girlfriend was 18. We dated for about 3 and a half months. Things were amazing in the beginning, we just clicked and enjoyed all of our time together. We had amazing all the time. About 2 months in she was out of town visiting a friend and things were still great but for some reason I had a feeling in my gut that I should look at her email. I know this is a bad thing for me to do but I did and I couldn't believe what I found. An email to the guy she was spending time with before we got together.. telling him how much she likes him/misses him and wants to talk on the phone with him all the time and such. But there were also emails to a friend telling how much she loved me and how amazing she thought I was and just crazy things like how she could picture marrying me and just how she was totally in love with me. I didn't understand. I ended up asking her about it and she got very upset..she cried and told me how she loved me so much and didn't want to lose me. So I basically let it go and forgave her and she promised me she would never do anything like that again..she told me that at the time of writing the email she was confused but she wasn't anymore..she knew she loved me. About a month later she told me about a guy she had met and got his number. She told me she had no interest in him beyond being just friends. I said ok that's fine. His name came up a week later and out of curiousity I asked her what his job was. She got quite defensive and asked me, "why does that matter?" I was like, "wow I'm just wondering..it's no big deal." So I thought that was weird how she got defensive about it. So anyways, I end up peeking at her cell phone one day (I know, BAD..i'll never do this again in a relationship that's for sure.) and I find that she has been texting him late at night.."hehe*blush*..maybe you'll come into my dreams tonight" or something along the lines of dreaming about him. I broke up with her on the spot after seeing that. We got back together maybe a week later and I thought I could forgive her. But it didn't last..she began acting strange telling me she didn't feel the same about me and felt distant. She told me she no longer had any drive. So we're broken up now but I feel like she almost turned it around like it was my fault or something. Did I make the right choice when I broke up with her in the first place? She says that I'm insecure and she only wrote the text to this guy because she was mad I didn't trust her. I always thought that would be the last thing you would do if you wanted to earn someones trust.. Anyways..that was long. Any thoughts?