You're not the first guy this kind of thing has happened to, and you certainly won't be the last!
Currently, I have a similar situation, but reversed. I'm not married but have been with a girl for a few years. My work situation means I travel a lot and this has made home life pretty tricky, recently I have found myself attracted to a girl I work with and I have reason to believe she has feelings back. These situations are never easy because ultimately, which ever way you slice it, somebody gets hurt.
I hope it works out right for you.