Sleeping with a married woman

I have been having some mixed emotions last couple months. It all started when a friend of a girl I work with came by and I was extremely attracted. This girl starts coming by more frequint at the work place just hanging out helping a little. Of course she is my dream girl and is somone elses judging by big shiny rock on her hand while I have a GF that lives with me.I really enjoyed talking with her, we would have great conversations cause I beleive I felt like that's what we had and all it would be. She seemed well off and doesn't have to work and seemed to have good stuff to say bout her housband. I was leaving out of town for work, she was helping me get ready and take care of things for me while I was gone and wanted to do lunch when I got back. Two lunch dates and she invited her self over when it was quiet at my place. Needless to say it's getting out of hand. Having sex few times a week. We meet for breakfast or drinks and food at early eve or lunch and even did a sat.night formal date followed having sex in my car all night. Bottom line I'm wanting to run away, I haven't felt as into a girl in a long time "I'm 32" not sure cause I will probably never really be with her. The guilt is starting to pile up as well, she has been well provided for and has a nice life style &things but she thinks she is not happy, sounds like he doesn't treat her well, discredits her and be littles her.
The sex is getting too good, dates are too intimate and full of affection, it will
not be easy for me to quit. Not hard to tell I'm developing real feelings for her
and beleive it's mutural by her actions and comments. Need some input people
I want to run before her marriage is destroyed by bad choices. I don't want to
ruin a married couple life, that would be hard one to swallow.