I had sudden onset of many disturbing symptoms about two months ago, brain fog being one of them. This, as well as my other symptoms, have been so severe that they greatly diminished my quality of life. I've looked into all the theories I can find including candida, heavy metal toxicity, sluggish liver, adrenal fatigue, etc. I've found a wonderful N.D. who is helping me sort out this puzzle. We've narrowed it to four general problems. The one, though, that your symptoms sounded like was my recent dx of hypoglycemia. I know you said that you've been tested for it but my Primary didn't pick up on it either. My N.D. came to the conclusion based on a bigger picture approach (the best approach) and so far it's proved accurate. I too have problems tolerating stimulants anymore, and have issues with anxiety. As I monitor my symptoms I believe I may actually have reactive hypoglycemia. I've included a few links about the subject. I hesitate to give advice because I don't believe in a one size fits all "cure". I will tell you though that the more I put into practice what I've learned about regulating my blood Sugar and what works best for my body, the better I feel and the more control I have over the brain fog.