well, anyways, I think you'll have to open your eyes and realize that the grass is always greener on the otherside, and you're going through a midlife crisis.
What she is to you is your pre-children youth... when things were easy and times were good. Because you were so young, you we're probably a bit green in what really is love, but as you got older you found what true love is, but you're younger idea of love still exists.
What I'm trying to say is,
1. You have kids and a wife who's had kids, you long for the days of
ing all night, as loud as you want with a tight bodied woman doesnt ask you to tuck the kids in or take out the trash.
2. You're getting old, and its freaking you out. And you havent had a chance to play the numbers game and realize life is still young. You're still thinking like a teen, and 30 is Old.
3. Grass is always greener, especially when your married, and even harder if the marriage is turning sour.
What I would suggest is that you cut your losses now, and realize that what you had... will never be again. The mind can play tricks on you, but the harsh reality of it all is that you have kids and she has kids, and in the end, one of you is going to not go through with it and the other will be screwed to the point of no return with their husband/wife.
It seems your marriage is falling apart, and/or you're falling out of love with your wife and you're looking for a way out. Be a Man, and go to marriage counceling and try to work it out. There's a reason you married your wife, and not this other chick. It was her loss... now move on and try to save your marriage. And after Christmas/Valentines day things arent working out, start figuring out your Exit strategy. Just remember, there's a reason this didnt work out in the first place, and this chick was playing both sides (like she is now). Also, you said you prayed for her, good for you, but if you're a religious person, you should concider this to be God plan? Its always easy to overthink about the one that got away.
But never screw over the woman who will take half and your kids away from you, stop being selfish and realize that if you stop talking to this woman now before it esculates, you can keep it your secret and save your marriage.