I've decided to break it off for good with my gf. How should I approach this?

I'm going to break off my relationship with a girl that I have gone out with on and off for 2 years. We got back together in January after she (mostly) convinced me that she got her life together (she started doing heavy therapy). I gave her the benefit of the doubt and we began our relationship anew. One of our problems was the fact that she was never got over an ex. The guy was a total cheater/creep but she still kept in contact with him. Anyway, she promised that she cut all ties, etc. I said if you ever get in contact with him that it was over and i made her promise that if she contacted her that she would totally ignore it or change her number/email. ANYWAY -- taht didn't happen. I found out because she has been distant recently like she was in the past just before we broke up (it's our sick pattern). I sensed it immediately and knew something was up. Here's the bad thing: I checked her texts. It confirmed my suspicions. They aren't sexual texts, but she obviously has been keeping this from me. For me, the relationship is over forever (yes, I know what youre thinking). And I know this is a similar question as I posed recently. I want to get opinions on HOW i should break up with her. Here's the rub: we had a little scare this week regarding a possible pregnancy. She took plan B but we're waiting to see if it works. I don't want to abandon her right now because i'm the "nice" guy, but I also want to make sure that I'm not stuck with her forever because this girl is a lying rat. So, how do i break it off? I have to wait to make sure we have the "all-clear" but should i tell her that i read her texts and know what's going on? Or, should I just tell her that it's over and break it off?