I don't know your story or how long it's been since Dday. Or how long your wife's affair was, but I can give you my perspective as a former wayward spouse.
It could be she's doing as RW mentioned above, she's spending too much time looking at the grass elsewhere and thinking it's greener, as opposed to keeping her mind on track with your relationship. If she had a big emotional attachment to the OM it will take her some time to really see that relationship for what it was. And to stop wandering what she's leaving behind.
It also depends on the reasons for her affair and if she's now committed to making your marriage work. There are several factors involved as to why she can't move past her feelings for him and not have contact.
Learning to concentrate on where she is today is a big key. And it's a daily choice to stay on track. If she recognizes that she shouldn't be contacting the OM and she's taken steps to stop, then IMO that's the first big hurdle to get past. At least it was for me. I had to set my mind to a goal and then remind myself daily (sometimes hourly) what that goal was.
Sometimes a counselor can be a big help with this. Someone you can run your fears and insecurities past who will not be hurt by what you are saying and feeling can be very valuable to talk to. My counselor helped me realize that my "urge" to contact the OM had really nothing to do with the OM himself, but had more to do with my insecurites and fears. And I was reacting to the things I felt I was getting from him. I didn't have a great love for him that drew me to him. I had a great "need" to get what I felt he was providing. Learning to get those things from myself, or my husband helped me get over the urge to contact the OM. He became a non issue when I learned to look within myself.
Hope this helps. If you'd like to ask more questions, please do.
GT