Don't let her make you feel guilty. That's where your power is here. Just say, "I'm sorry you feel that way" and walk off. You know what's going on and why. You know what's better for the kids and its not having to grow up with an addict mother. You know the journey you face will be a hard one, but one with a light at the end of the tunnel. Her journey will be far more painful than yours...
Stay strong and firm, you'll be ok. It sucks, really really sucks, to be in this situation, doesn't it? But everyday away from the addict is a happier and more serene day, I promise you that.