My fiancé had an affair with his ex wife..

My fiancé and I have been together for over two years, except for about 2 months when we broke up. After asking me to come back to him he proposed and I moved in and we set the wedding date for May 2013. During the time he and I had been together before we split up I had always had a feeling something was going on between him and his ex wife but it wasn't until after we became engaged and living together that I found out that they had in fact been having an affair for almost the entire time of our relationship. Needless to say I was quite devastated. Once the ex wife found out about us planning on getting married and me living here she started causing a ton of drama. She kept his kids from him and they fought constantly which put a lot of stress and tension in our relationship. As the date for us to get married arrived he decided we weren't going to be getting married "bc of our fighting" and how upset I had been over his change of attitude towards me like I all of a sudden was the enemy. During his last days home(he works in the oilfield and is gone for half the month) I noticed him and his ex were back to getting along and she wasn't starting problems and was letting his kids stay at the house again. I immediately confronted him and asked if they were back to being up to no good hence his change of mind about getting married and them back to being bff's again. He claims to not know what changed her mind/attitude and she has a boyfriend now and there's nothing to worry about. My question is how do I get over the hurt of being cheated on? I want to confront her so bad and let her know that I know all about the two of them but idk if that would be the right thing to do. How do I know it's not happening all over again?? Any advice would be greatly appreciated, thank you.