Lyme (disease, not Regis)
The great news (oddly fantastic, I'll admit) is that this appears to be Lyme disease, which is treatable. It all started a year after I began backpacking. I never knew I was bitten by a tick, but turns out they're often the size of poppyseeds, so you don't notice them. The tests for Lyme are unreliable and the disease is incredibly good at masquerading as something else, or hiding completely.
I'm so glad I didn't accept my family doctor's "It will probably always be a mystery why you feel bad." Turns out, anyone who's been diagnosed with chronic fatigue or fibromyalgia should find a specialist who knows how to test for Lyme. (Even my infectious disease specialist didn't catch it; if they don't talk to you about CD-57 counts, they may not know what they're talking about.)
In a few months, with antibiotics, there's a very good chance my symptoms will improve. I can't tell you how thankful I am. I'm overwhelmed to think healing is possible.