I went through a similiar situation about 2 years ago. I thought my almost 18 year marriage was fine (honestly didn't have a clue that anything was "amiss"), but my husband was living a double life (including a young girlfriend)-- I was shocked (and devastated) at the time.
I wanted counseling, and we started immediately. He said he was sorry, etc. etc. etc. I honestly thought we had hit our "bottom" and would climb our way out together . . . fast forward two months, and I discover the affair had continued. He left . . .
You need to start NOW to get the ball rolling to protect yourself and the children. Get a calendar and start documenting (everything)- you going to counseling, etc. I would DEFINITELY get a P.I. NOW. Here in the state of Virginia, you do NOT have to pay spousal support (alimony) if the cause of the affair is adultery or abandonment. Keep "playing the game" while you are getting your ducks in a row. If you have money in a savings account, I would draw out half (no more than that) and put it in an account in your name ONLY so she can not have access to it. Start MOVING NOW!
Remember that anything you say out loud or put in writing (on the internet, email, text, etc.) can be admissable in a court of law so do NOT put anything out there that you would not want to have to defend in a court of law. I am not saying you are doing this, but just be careful!
I KNOW firsthand how raw and emotionally draining this is for you. I KNOW how devastating it is. It is even worse for the children. Do not do anything or say anything negative about your spouse (even if she does)-- it will not help the children. Both my (soon to be) ex and I have done very well with this. He is a good father. I would LOVE it if he would take this house (and the high mortgage payment) with 100% of the equity it will bring and get my name off it so I could just "start over". I do NOT like having to have spousal support in order to provide for my children. Of all people in the world to depend on, I do not like that it has to be him. However, he will not do this, and so I do get spousal and child support. So there it is . . .
Now my road 2 years later is vastly different . . . I am stronger than I ever thought, I am a better mom, and I know I can stand on my own two feet.
If your wife is set on leaving, hold your head high and start making a path for yourself. But, start planning NOW!!!! Do not wait for her to act (she probably already has!)
Please, really consider that P.I. I wish you the best of luck and offer my support and prayers!