I think this is one of those cases in which you madly desire to take action in order to fix something, but deep down you know that the right thing to do is: "do nothing". And by doing "nothing" of course i mean to avoid any attempt to try to convince her.
In this situation is clear that right now, she is happy with other person. So trying to convince her to be with you instead of her beloved, woud only make things worse.
But i think there is still hope by pretending nothing happens, things are cool and you dont need her, but most important, make the sensation real by continuing your life and perhaps: meeting new girls?. Make her know you are happy with no hard feelings, keep her as a friend under a cocky & funny/ tease & neg base and her attraction to you will remain on. (There is no need to express remorse to her for "being so stupid" when you stopped things at the beginning. She might and probably will have a problem with her partner someday and the chances that she will look for you are not so "farfetched". (and yes, as you know, perhaps by that moment, you wouldnt even have the same"need"for her as now). But try not to wait for her, continue with your life above anything.
On the other hand you must not feel bad for not taking advantage of the situation at the beginning, cause as you said, you didnt want to be responsible for a messy breakup with your wife. Now you can place all the blame on her. Enjoy your good conscience
Warm Hug.
Paolo R