Why did you end up leaving S for OP and or why did you stay with S? And did guilt have anything to do with not staying with your S after the affair? How did you justify the A in the first place? Were you truly in love with OP or tried to convimce yourself that you were? And last but not least what could your S have done to stop the A and win you back? [color:"purple"] [/color]

I loved her. She is a great person.
No. I would never have stayed in the M just because I felt I "owed it" to my W. I do feel guilty about the A.
Here are the random thoughts from that time: My S doesn't understand me. We have drifted apart too much. She leaves me alone all the time to visit her family. I want to be around someone who is interested in me. OP makes me laugh. All I ever do with S is fight.
I was "in love", but not with OP. I was in love with the illusory person that I had created around OP. I didn't know OP. I spent perhaps 5-10 hours with her a week at most, and most of that was in bed. So, I didn't know her friends, family, etc., and she didn't know mine. So, there is no way that I could be "in love" with OP. It would be like a 5 year old saying, "I love Minnie Mouse."
Her willingness to make the M better was the key. She actively helped make the M better. BUT, IMHO had my W done the things suggested on the board (e.g., exposure, NC letter, Plan A, etc.), then the A would have ended much earlier.

I was in love with the way the OP made me feel about me.

Appreciation - Interest in ME.
Communication - Interest in what I thought and had to say.
Time - Interest in spending time with me and being around me.
Attraction - I felt desirable and attractive.

Just like dating. Everyone is on their best behavior and putting their best foot forward. No nagging or having to clean, cook, do laundry or take out the garbage.

It is a fantasy life until reality comes crashing down because guess what is missing ~ R E S P E C T

All of these things are things my husband and I learned to foster in our marriage.