It sounds like the problem is just as much that your mom can be selfish as it is that she's spontaneous... just my reading of things, however.
Anyways, I'm also sort of in the middle here. If I'm not doing anything, things can be up in the air, and supposing that I know HOW to do something on a moment's notice, I'm not bothered by just deciding or having to do it out of the blue. However, it drives me absolutely insane when I'm in the middle of one thing and someone stops me to do something else suddenly. I'll switch tasks if I want to, but being made to do it is quite disruptive.