I was suffering from extreme fatigue last year, and have had everything checked out: heart, lungs, intestinal tract, blood, the whole nine yards. They couldn’t find anything, so in the end the conclusion was that I have long covid. There is no way to definitely diagnose long covid, though, so it’s kind of a last resort diagnosis.
I’m doing a lot better now, mainly thanks to a low/mid intensity training program especially for long covid patients. I’m not nearly back to normal, before the fatigue started, but a lot better than I was one year ago. It’s all relative….