Perhaps related: Illness (with effects similar to fatigue, or worse). Particularly frustrating when it's chronic and without a ready remediation.

I don't mean it as a cop-out. But rather, that if one is experiencing such a problem, it needs addressing. Overcoming it through sheer willpower makes for a good TV movie but can be counter-productive when taken on as an objective, personal measure -- especially when others are echoing Nike's "just do it" in lieu of actually helping fix the problem.

Lose the slogan, and put getting healthy at the top of the list. If your medical provider has nothing to offer, keep looking.