Perhaps fatigue is not limited by energy but by the byproducts of energy usage to do work. It becomes much more complex to tackle if that's the case, instead of finding ways to allow the brain to use more energy when available we would have to mess around with the cleanup processes which are much less well understood.

I don't think we will have a way to "cure" mental fatigue until we more completely understand both the mechanisms behind thinking, as well as resting, and at the moment we barely scratched the surface of them.