We can always have a soft spot for our Exs. There must have been something about them that we liked in the first place, to have even dated. So if you reminisce about the past, that is okay for awhile. But remember, that is already in the PAST.
Love is also a commitment, a conscious decision. And not just a rush of feelings or the passion of the moment. Read here
Attraction Mind Map » A Note About Loving Consciously This Valentine’s Day
You've got a wife and two lovely kids. Sure...your wife is not Mrs Perfect but neither are you flawless. Be appreciative with what you have. My analysis may be different if you have said that you do not even have a happy family to begin with. I also think that this is the case of the grass always looks greener on the other side.
If you love your kids too, then think what happens if you tear your existing family apart? My suggestion is to wish your Ex well and move on. If you need to stop seeing her for awhile, then do what it takes to have the clarity of mind for the right way forward.
All the best,
Evelyn