Oh, gods, this has been an ordeal recently. It has been frightening and frustrating both. At points when the brain fog was at its worst, I was re-reading Book 5 during the editing process and realizing that I had forgotten writing entire chunks of chapters.
I can’t necessarily give advice because I’m not an expert; I can only talk about my own experience and conclusions.
First, I realized that if the brain fog is really bad, then that means I need to stop trying to do the complicated mental work and either do some easier admin work, or simply just rest. Yes, this means that I can’t set deadlines that don’t allow me that leeway. (Though I still have deadlines–for Book 6–that I set up before my diagnosis and at this point cannot push out any more due to Amazon’s rules.)
Secondly, I have to start taking better notes. This is part of what I’m doing a re-read of the series from the beginning for. I’ve naturally forgotten details since it’s been years since I wrote the earlier books, and I’ve been dealing with brain-fog more recently making me unnaturally forget details. While I read through, I make a note of everything that sparks a feeling of “I could make a call-back to this/I could repeat this for character consistency/this could be a relevant conversation point or plot point later.”
I’ve already, since the beginning, taken extensive notes about worldbuilding, character stuff, etc. But I need to do a better and more thorough job of it, and make sure that everything is organized in a way that’s really easy to retrieve information. I’ve also got synopses of everything that happens in every chapter, which I think maybe could be expanded a bit. Again, all this takes extra time.
The other part of taking better notes is making sure I have a list of the things I need to do. When I make a plot or character decision/revision, instead of imagining that I will remember all 5 of the spinoff ramifications of this, I need to note each one of those ramifications down. Then I need to check each of them off as I actually write or revise them. (I have found myself thinking I wrote things that I didn’t write as well as forgetting what I did write.)
But the first point is still the most important. The healthier I am, overall, the less brain fog I have. The more I rest, the less brain fog I have. Getting rid of the stress of trying to do more also seems to help.
And RobotNerd, if you (or anyone experiencing brain fog or other unusual symptoms that aren’t exactly sickness) haven’t been to the doctor, consider getting some testing done. I don’t know your situation, but there may be some simple thing causing your brain fog that you could treat.