Thanks dcook. Out of my will power is what I wanted to say. It's just that I learned we have a resource pool in our frontal lobe that we use to make decisions small or big. It can be depleted, literally. Thus, the strange academic wording.
"Willpower" usually refers more to self-regulation -- it takes willpower to stay on a diet, to stick to an exercise plan, to keep a schedule, etc -- than making decisions. It can be easy to make a decision, but takes willpower to stick WITH that decision.
Decision-making is often associated with "reasoning" -- your reason for choosing X, etc.
(We DO say "I've lost my willpower" when we break our diets or give up on a difficult task.)
I don't think it would sound too strange to say "I just don't have the energy to make that decision right now" -- although it would sound like "I'm just too tired," it would also imply not having the mental resources to dedicate to the task.