It doesn't sound to me as if you are doing enough exercise to really wipe out your endurance like you are describing. I didn't see where you mentioned how old you are, but depending on your doctor and the protocols that your doctor is required to follow, not all relevant blood tests may have been run. Thyroid disease, for example, is supposed to be rare among men under 50 so some younger men have difficulty getting doctors to order blood tests for thyroid disease even though the symptoms would get a screening for thyroid disease moved up to the top of the list if the man was over 50. Undiagnosed Lyme disease can cause a bunch of similar symptoms also. If there's no possibility of exposure, then cross that one off. MS, as I understand, can only be positively diagnosed by MRI, and can be positively eliminated as a cause as well.

Think very hard about any other mental or physical changes that you have undergone in the last year or so. Are you more susceptible to cold? Is there anything about your memory or concentration? How about joint pain/stiffness? Susceptibility to depression can be caused by endocrine imbalances, too .Write down everything you can think if that has changed, even if you don't think it's important, and talk the list over with your doctor.

Sometimes a person can have some ailment like this and it will go away spontaneously and a cause is never found. So far, it doesn't appear as though you're that lucky. If you suspect a nutrition problem, or suspect that you might be able to fix the problem through changes in diet / supplements, talk that over with your doctor. If your doctor seems to be at a loss to help you, get another doctor. But remember, this is probably a medical condition that needs to be rectified and it could be very serious.