Detaching takes time and comes in stages. The fear you will conquer.
I set goals and break them down into workable pieces. At first it was as simple as I will read one chapter in a book (having nothing to do with R's etc) that requires my focus (I chose things like quantum physics, string theory etc). At times it was all I could do to read a page. I made sure I was doing some kind of physical activity (not work) again, you can keep it simple.
As far as what you want. Don't discuss it with her. Write it out. Then revise it until you understand what it is you want. If you don't want to help her with some request, you can say no nicely. Do you get out to do things? If you don't, start.
As far as the kids knowing you did everything in your power..you're right that you don't get to tell them that. They will come to know (and at their ages already do) more than you think. I am surprised and at times embarrassed at what my kids understand about everything.
If you can do something to work of some of the nervous energy from all this, you may be able to quiet your mind (if only for a minute or two) enough that you can think. My caution here, just because you think it with what you think is a clear mind doesn't mean it gets discussed with her. Put your own plan in action and measure yourself by your own yardstick.
