I'm sorry your brain fog is so bad, OP! Diet is the key to managing my own symptoms with this. I lay off processed foods, simple carbohydrates and especially sugar. During times when it gets really bad, I go on a ketogenic (<50g net carbs a day) diet. I have MCAS so this might have something to do with why it works for me, I'm not sure. But if you don't want to resort to such drastic measures, just reducing carbs and eating plenty of high quality protein and vegetables seems to help me and might be worth trying.