Just FYI, I found the Boundaries series of books to be literally life-changing.
TODAY, go get "Boundaries In Marriage", and read it first. I read the whole thing over a weekend. The more general "Boundaries" book is excellent too, just in a broader sense.
I know it talks about letting the spouse have the control, not enforcing boundaries, etc. And it covers affairs, be they emotional or physical. So, so much good stuff in that book!
You need to figure out what your line in the sand is. And trust me I know where you are coming from. It took me forever (almost 2 years!!!!) to REALLY grasp what it meant to know my boundaries and enforce them. I am still learning and putting it into practice.
You need to know 100% that you are not letting her run off across the country with your kids. She needs to know that if she goes, good luck with that, but the kids stay here with me...
Remember, you have been catering to her throughout your marriage, you said yourself. Ask yourself, HOW IS THAT WORKING OUT FOR YOU??? There is probably nothing you could say or do coming from a place of self-respect and healthy boundaries that could cause more damage to your situatution that what has already happened when you were living without any boundaries...having them lets you know what you are and are not willing to stand for. It gives you stability in place of the back/forth you are going through now.