There is an interesting book called Brain Energy by Chris Palmer which may be relevant. It takes a stab at identifying the root physiological cause of mental health problems. His hypothesis is that many psychological and mental health issues are caused by problems in energy usage and metabolism in the brain. The increased resistance to even minor tasks you describe sounds like a brain trying to conserve energy.

I don't think I buy his ideas wholesale. I'm sure there are brains with perfectly healthy metabolic health that are nonetheless wired in ways that cause a particular mental health issue.