I have extremely bad brain fog, especially around my period. My doctor indicated that she believes it's an influx of hormones that makes everything go so off balance. This is usually when my fatigue is at it's worst as well. She wants me to see a hormone specialist. I imagine controlling it with birth control would also help, but that isnt an option for me at the moment.
In the interim, the census was to go on a stimulant to assist with 'pushing' through the brain fog. It had to be a low dose, since we didn't want it to affect my heart rate, but just enough where most days I could manage the symptoms. Things improved once I had the perscription, but there will still be bad days. The process of getting there was also time consuming. I had to see my dysautonomia Doctor, and Neurologist, and then a Psychiatrist to finally get to the point where I could start taking it.
Sleep is also a major factor. If you're having sleeping issues, definitely bring that up with your doctor.