I also know what you're going through. He is probably seeking out something he's not going to get in your marriage: someone to drink with. My AH was always on the prowl especially when he had been sober for a long time, wanted/had relapsed, and needed a double life to maintain his "relationship" (for what that was) with me and his relationship with alcohol.
I remember how devastated I was, too. In my case, he had adamantly refused to go to a family function. I couldn't figure out why, but I said OK--happy I wouldn't have to drag a miserable mope with me. I even left my daughter with him. When I returned my daughter said something about "the lady" and I learned that he had THE NERVE to invite his gf to OUR HOUSE, go to OUR SWIM CLUB and bring OUR DAUGHTER with them while I was away.
I think I was more devastated at the blatant invasion than I was at any personal hurt. He had even removed the family pictures from the living room. Honestly, the sick part was that I didn't even really care that much about the gf--it kept him out of the house while he was drinking. But the fact that he could be so bold and disrespectful and hurtful floored me. As time went on, it also floored me that I turned a blind eye after that. But I was so determined to keep the family "together" that I put every feeling I had on a shelf in the closet.
Sorry to digress... but my point is, the drinking probably opened the door to the infidelity. If I were going through this again, I would get to a counselor asap to help me sort my feelings and then decide what kind of life I want for myself.