I can hear the pain and struggling in every word of your comments two posts down. I know how that feels-- to not even know what you are getting out of something, but still need it and crave it so much. I hear you saying a lot of negative things about yourself. You hate who you have become. You don't know how to fix your marriage because it is great already. Um, hello? If you are hating yourself so much, then obviously not all is OK with you.
Sure, your DH and you have a good sex life, but that is not everything. Look inside yourself and ask why you were open to those MSN messages when they started. They made you feel good about yourself, right? So obviously you need some help feeling good about yourself. We all do. For me, what pushed me over the line between flirting with someone and really getting physical with him was that I still hated myself deep down, and I was looking for outside validation that I was OK. My DH contributed to my feelings of not being OK with myself, and having low self confidence. But they were around long before I met DH.
Through therapy, I found out just how upset about my own self I really was. I found out I had deep shame about the person I was... something I was attributing to the way I was raised, or having a mom who was manic depressive, or not growing up in the right kind of town, and being too blue collar. There was this real strong soundtrack in my head telling me all the reasons I was flawed, less than, not important, stupid, ugly, worthless. When I met DH and we knew each other long enough to start being honest with one another, his reservations about me became clear. He didn't like the dark circles under my eyes. He was worried I would get fat. Charming, right? But I can't blame him. I could have walked away at that point or just told him, eff you! I'm not fat and I was born with these eyes! Take them or leave them! Instead I silently let those opinions confirm my own inner hatred for myself, and took it as another piece of proof that I was a wretched little thing.
When AP came along, I perceived that he accepted me more, "got" me more, would make me whole in some way. He didn't. I too think he was just in it for the sex. So for me therapy helped me realize that AP was not going to do all the things I was subconsciously hoping he would do. I had to start repairing my insides, not look to someone else to do it.
I strongly recommend you get into therapy again, and your one friend that knows? Can he/she help you give up AP? It is seriously like drug addiction-- you need people to help you or you won't stop. Best of luck, I have to go... I'll try to check in as I can.