One of the things I've done when I've had problems has been to do innocuous experiments on myself. Supposing it's a mitochondrial problem, regardless of the cause, one could try a little self-diagnosis by stimulating the mitochondria with supplements:
N-Acetyl-Cysteine (NAC) 600mg 2/day
Acetyl L-Carnitine 500mg 2/day
CoQ10 100mg 2/day
PQQ 20mg 1/day
All on Amazon.
A month of that either would make some difference - or not. Cheaper than doctors or doctor's tests which don't show anything abnormal. If that does make a difference, back to the doctors to find out why.
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Results matter