Brain fog to ME (but may be not for you)is like a kind of depression, being tired and no energy. The hardest thing is can you solve it, it is not easy when you don't know the reason. I just kept flushing because it helped the fog.
Then I found it often happened when I ate certain foods, so that must be allergy related. So after 2 years I went on more low carb diet and that for a while, several years, worked. Then I made an adjustment again less meat and more veggies and not much fruit. Now I wait and see, I just don't eat bread and pasta etc and always try to do food combining. Protein with veggies and no carbs. Eat fruit by itself without any other food.
It is sometimes an endless journey but always interesting.