How do I get my girlfriend to stop talking to another man?

My girlfriend and I have a little boy together, and this is what makes it so hard on me. She says that I am controlling and jealous, and the whole nine yards. I believe I am only that way because in the past she cheated on me with her ex. Now she uses the excuse that I am being an ass and that I am insane to keep looking through her stuff, to see if she is being faithful. But the thing is you can tell if something is different about someone, they start acting weird. One day I went on the computer and she left all of her emails and her a social site open. Im not gonna lie I glanced at it. When I did I saw an email from the guy she told me she wasnt going to talk to anymore. Now this guy lives 1000 miles away but thats not the point. In the email in so many words she tells the guy that she is miserable with me and that she wants to have sex with him. I saw this and confronted her about it and she flipped. Okay I looked through her stuff but we have been together for almost 5 years we have a kid and I care for her other two kids. I have never cheated and will never cheat even if it sounded like a good idea. We even got into a huge fight like 3 months ago and she told me that if I keep it up she will have him move in and have my son call him daddy. I cant controll myself because of the things that she said. I dont know how to stop myself from being the way that I am because I am in fear of seperating my children because I know what its like, my parents got a divorce and it sucks. For the past week she has been good as far as I know and I really havent checked her stuff at all. I came clean today about what I did 2 weeks ago with her. I went on her a social site that was already open, looked up that guy, blocked him and deleted him. Now I dont know if she was flipping today because I went on her a social site or if I deleted him. Someone please help me with this situation, for the sake of my kids.