Yes, @Blueowl, I've had a very gradual decline over many years like you describe. I think in my case it's due to Long-Haul Lyme disease.

Severe stress and emotional upset can trigger an inflammatory response. I think you should take a long break from regular workouts, which is just more stress, and get plenty of sleep.

I agree that you should visit your PCP to check on cortisol, thyroid, iron, plus the usual. If you can afford a Naturopathic doctor (or one is in your insurance network), get an opinion from him or her, too.