Sorry to hear about your situation but glad you have decided that your marriage is worth working/fighting for.
First thing you need is to have patience. Give things time. Him deciding to have an A probably wasnt a decidion that happened in an instant and will not be solved in an instant.
The key to this program is to work on yourself and not him. Start eating and sleeping good and find something to do for yourself and your PMA.
Are you sure you want the details right now? Don't seek answers to questions that you really don't want to know the answers to.
When my W started her relationship with her OM she insisted that they were only "friends" and didnt spare me any details but I knew that there was more...turns out yelp there were more. But from my situation, I would be closer to reconsiling with W if I didnt waste any of my time with the details of their relationship. I felt helpless and betrayed as well but made the mistake of showing it and that pushed my W farther away. If I just let things be and worked on us instead of worring about them I would be in a totally different situation right now.
