You didn't do this to get back at her. You did this to make sure you still have financial stability that you need to take care of you and your family.
AGREE WITH THIS. ^ And this is exactly what you tell her (and yes, tell her proactively -- don't want for her to find out). Your position is that you did what you had to do in order to protect yourself and your kids, and because you'd decided that you were no longer willing to financially enable her waywardness/walkawayness in any way. So if that's your position (and it's the correct one), then you have nothing to be ashamed of.
Tell her calmly, matter-of-factly, and business-like. If she is livid (and she probably will be), just say "I understand, you're angry. But I did what I had to do in order to protect myself," and don't waver from that.
