Now when it comes to focus, I can lose it quickly too, but if I really am in a good mood to work, then things mostly work out fine.
I have heard about CFS and you are not the first I meet with this syndrome, but I'm afraid I know almost nothing about it, but since this has little to do with development in general, but rather with a medical status, perhaps the best way to go is to medics, but also to people who suffer in the same way you do. I suffer from the Asperger syndrome myself, and speaking to fellow Aspies (and one of my ex-girlfriends being an Aspie as well) did help me a lot to deal with loads of things. And this basically goes for all medical conditions. Speaking to fellow-condition-havers can always be by fare more enlightening than speaking to medics does, and this goes for both mental as physical conditions. After all the only way to fully understand what it is to have CFS is actually suffering from it, but even then there might be aspects of your own condition you may not yet fully know or never thought about, and that's where people who suffer from the same condition can sometimes have good pointers. Of course, every person is different, so what works for them may not always work for you, but at least listening to how they cope with things, can sometimes give you some good ideas for what may work for you. It's one of the ways in which I explored the Asperger syndrome. That's the only advice I can give. It does not matter if what they do has nothing to do with game development, as you may still find some things they do to make what they do possible.
Now perhaps as somebody who has the habit of sometimes drinking too much coffee, I may not be one to talk, but drinking caffeine solely to stay focused is rarely the way to go. I prefer energy drinks only when I need to drive at night, to make sure I get at my destination in one piece, but since I rarely have to do such rides, the danger is minimal, but in your case I guess things are different. But if there's really no other option, you may wanna discuss with a doctor how you can go this in a relatively safe way, or if there are any alternatives. It's mostly not a good idea to see medical advice on an open forum.
And with that I've given all the advice I can give, I'm afraid. I don't know how many people you may find on itch who suffer from CFS, as well. Perhaps you should look up if there are any places where people with this syndrome seek each other, preferably in your neighborhood so you can meet them IRL (which often works better than over the internet).
I really wish you all the best.