Puppy I can relate to exactly what you are saying. The lies were the worst part your sanity jumps from minute to minute throughout the day. My H and the OW who is also married were not going to admit to anything. Out of straight exasperation and not wanting myself, my kids and my mother-in-law to go through this any longer I finally exposed the affair to her husband. He already knew as well just didn't know who. Unfortunately that sent them into each other's arms and took it from and EA to a PA. However for me the EA was ten times more damaging than the PA portion. Like you I didn't want my marriage back I just wanted my sanity back and to not be viewing as a paranoid crazy person any longer. Also, like you once he couldn't lie anymore he came back home and has been working diligently at putting our marriage back together. I don't know that I will ever know the answer to "why the lies?" but I do know he did love me but as WDID says he was caught up in the fantasy of a perfect relationship and couldn't break free.
