I have a great perspective for you. Me, 30/f and my bf 44/m, he's basically my age in his head... not the point. But many times I've been contacted by exes and former casual encounters and I'm well aware of how this affects my BF because in the beginning of our relationship I didn't think there was anything wrong with communication as long as I didn't take it anywhere. I went to lunch with an acquaintance, told me BF and he flipped because he assumed I was telling him that I fell in love with this acquaintance. I didn't ever see the lunch date again, and I had no intentions of doing so. I just wanted to meet a new person. But my honesty in the situation led him to become gravely insecure. I thought him crazy but a few months later a former "lover" contacted me to chat. I was a little pissed at my BF so I didn't tell him about the texts. However, I mentioned my BF to the former lover in the texts and the tone of his texts changed. He was looking for some booty basically and the enthusiasm I displayed for my BF to the former lover, via text, changed his attitude because I sub-textually told him I wasn't available. So my point is I think it is wrong what your GF did 4 years ago, but, I would not tell her what you did. DO NOT tell her. This maybe a lesson for you. But to me it sound like 4 years ago, maybe, she felt inappropriately flirty and wanted to relive some time in the past through this ex-lover. If she ever does it again, dump her. But sometimes we ladies lapse because we have commitment issues, self esteem issues... etc. I looked at my BF phone once to find a woman crazy in love with him. However I didn't know, but he had zero... ZERO feelings for this lady and just didn't know how to break it to her. ( they were former lovers 1 year b4 I dated him) I felt guilty like you so I told him what I saw... and he was pissed. I don't think his pissyness was warranted but the trust I had to build back because he didn't want to tell me about crazy lady was too much man. learn to keep the secret. it isn't a terrible terrible thing you did. She could have deleted her shit but she trusts you. Let her keep trusting you because building that shit back is rough.