Nothing leads to a person breaking their marriage vows except deciding to do so. It is called "cheating" because people wish to have their cake and eat it too. My husband told his wife he was dating me (before we went out) and moved out of their home as soon as he could arrange it so she was well taken care of as well as himself.

The situation you have described cannot be easy, there is no quick fix or chart/map of what to do. I would keep talking with him. It sounds to me like he wants to be with her so I don't see where you fit in in that scheme? As hurt as you must be, I would also be insulted/angry that he is going about his telling and breaking up of the marriage in this fashion. He could have done it like a mature man and not "involved" you, giving you more pain.

You may have insisted on the details but they are his details, his affair, and he effectively betrayed the other woman, giving her details/history too to someone outside of that relationship. Their relationship is none of your business other than to decide, with him, how the fact of it impacts yours. He does not sound, to me, like someone I would want to put a lot of faith and trust in at this time or like someone who I could be assured cared for me as a husband should, or who cared for our marriage with the priority I believe it deserves.