I have had severe bouts of flat affect and fog. I've also bounced back as well. Focus is a struggle of mine and this plays into brain injury region I believe.

What causes those for me may be different than you.

#1) Nutrient levels

I found that once I started incorporating NAC, a precursor to the antioxidant glutathione, I could "feel" again. I also do well with fish oil, vitamin C, vitamin D, niacin, b vitamin complex, and lion's mane mushroom.

There's a book called "Why Isn't My Brain Working?" and in the book, the doctor says that brain fog is essentially caused by brain inflammation. He discusses how he reduces the inflammation/fog in the book with flavonoids.

Once our brains are injured, nutrients are used up and sometimes we need more than pre-injury to function.

#2) Food allergies and brain insulin resistance.

If I want brain fog, I eat carby foods. My brain clarity falls dramatically with wheat and dairy based foods as well as sugary and high-carb, processed foods.

#3) Emotional. This affected me early into my concussions and thankfully not anymore (that I perceive). but the shock of losing abilities and identity was a major player into my numbness at the time.

I find intermittent fasting can help with clarity as well as coffee and tea, but I crash with those, so I keep that in mind. Exercise can get some blood flow going as well.

Sometimes things such as candida or other infections can be in the brain and result in fog.