Wife Having Phone Sex

I recently found out that my wife of 17 years had been having phone sex for a couple of months with a lifelong male friend of hers. I found a couple of videos that she had made that she may have sent to him (one a bit racey and one saying that maybe they could get together sometime). Since I questioned her about the videos, she admitted to the phone sex but says she never sent the videos to him. She seems very remorseful and says she never meant to hurt our marriage. She also says that she has let him know that there can no longer be any contact between them because she values her marriage too much. We went to counseling two times and it seems to come down to me accepting that it happened and that it will never happen again and just moving on, I guess. We both have had previous marriages in which our spouses cheated on us and had conversations long ago about how much that hurts and how we would never do that to each other. I am having an extremely difficult time forgiving, trusting, and moving on from this event. We are getting along fairly well lately and continue to act as a married couple including hugging, kissing, sex along with the daily routine. It seems like she is past it all, but I think about it almost all of the time (almost). How serious were her actions and do I believe her when she tells me it was just phone sex and that it is over? Trusting her again is so difficult! Suggestions?