We have a lot of the same symptoms, the brain fog, the weakness and shakiness, nausea, never hungry or thirsty, and constant fatigue.

Turns out, after going to a gastroenterologist, I have an extremely high concentration of acid in my stomach. I never noticed because I don't have classic signs of reflux such as heartburn or acid coming up my esophagus but I did have mild chest discomfort and cramping.

He put me on some Omeprazole and supplemented it with some Zantac for a few weeks because it takes half a month to start working and, POOF, brain fog gone, weakness gone.

I'd give it a try as you don't have much to lose. I hope you find what's wrong, I sympathise with your suffering and I'm sorry you have to go through that.