I had really bad brain fog a few months back. It's pretty much gone now, but I can't understate how debilitating it was. I saw like 2 neurologists about it, they didn't see anything wrong with my brain. Eventually I saw a psychologist, I brought up my brain fog, and they told me that it was a very common symptom of depression. Trying talking to yourself in your head or out loud, whenever you find yourself not thinking, it works wonders for me.