It is human nature to want to know more golfwidow53 - even with things as hurtful and as tacky as this.
Part of why this happens is part of the grieving process which takes you through all the hurt, betrayal, reasoning, the asking of why, etc, etc, and, of course, the anger. In order to 'get it all out' you need to know more so that you can exhaust all the possible questions.
However, there should come a point when you simply have enough and either conciously or subconciously you will draw a line in the sand. What you decide to do at that point is, well, only time will tell.
From what you say it does sound as if my theory was right about the OW being scorned that he did not leave you for her - sounds like the details, locations, etc, that she has been telling you have indeed been designed to twist that knife of deceit further in your heart and hurt you. If she can't take him away from you then perhaps her thinking is that she will destroy the marriage or destroy you.
You have been terribly betrayed and are hurting but, as others have said on here, others have gone through similar things and come through it.
Yes, you may well decide to divorce him eventually but you may also decide that there is something worth fighting for. Only time will tell.
Best of luck with the counsellor on Thursday.