<<It all shows how much more involved she was with OM than she let on. No wonder you couldn't save the marriage.....>>
Sadly OD, this is par for the course around here....we just do not want to accept it at times. WAS are getting something from OP that they do not get from us. Now whether that is sustainable in the long run and whether you will be around if and when their relationship falls through is up to you to decide.
I feel for you....the kids meeting OM and the key...don't let it get you down. D|on't dwell on it too much. Surround yourself with people who love you and move forward. In time, what she does or does not do will become irrelevant or at least will slide off your back.