Should I read this?? What would you do?

My now fiance and I have been together for a total of now 5 years. During this time, like most relationships, we went through very difficult times. We were also very young and still learning. There was another girl that I was always told was a friend at the beginning of the relationship but she ended up being a big problem for us. Fortunately we have gotten over it but she's still a VERY sour subject and I feel in my heart at times that there is something I still dont know. Kinda like I was told what i wanted to hear kinda thing. anyways. When he was visiting his mom about years ago, he was chatting with this girl on AOL instant messenger ( a convo i was to know nothing about, as kinda like, Have you talked to her lately? and he would say no. So i was lied to in the past about him talking to her) anyways. He ended up leaving to go somewhere and his mom found that convo and knew a LITTLE about what was going on so she printed it out. She has told me about it 2 years ago and said......