I am seeing a married lady, and have been feeling guilty..?

I have been seeing her for about 3 weeks, and met her from an ad I put up under casual encounters in the personals on craigslist.. I know if she hadn't started seeing me, it would be someone else. But in choosing to be a part of this adulterous relationship, I know I'm doing something wrong. It's tough though, cause we have the most amazing I've ever had, and talk about our lives like best friends. Am I hurting myself subconsciously, by trying to ignore the fact I'm doing something morally wrong? And if her marriage basically seems to be at it's end, why should I beat myself up and feel guilty at all?