Is my wife cheating on me?

A while back my wife was texting this guy back and forth. I found out and I saw some pretty intimate exchanges. I confronted her with it and she swore up and down that she never cheated on me, and told me that it was just fliting, but I had my doubts. We were ok for a while but then the betrayal of remembering the exchanges that they had makes me doubt her. Now I constantly go through her phone and emails because I can't trust her. Our relationship hasn't been the same and she tells me I need to let it go because is going to ruin our marriage. Recently I found a couple facebook messages but nothing compromising. Apparently she has been talking to this guy from her work phone, on occasions. I don't know what to do she tells me she loves me, but it seems like every time we get into a fight she goes to this guy to vent her anger, at least that's how I feel because every time we get into a fight I check up on her messages and find phone records with him. I don't know how to confront her again about the facebook messages because she gets very defensive and says that I jump to conclusions. What should I do? Is she cheating on me with this guy or has she in the past? Sorry I couldn't be more specific but I would like some input.