I am sorry you are having to find a way through this. Sadly the only way to the other side is through. I remember this brain fog, it was like I was fine in the morning for a few hours then just ran up against a wall.
This continued for a while after radiation was done, I did start to have a lot of physical fatigue about two thirds of the way in. This all lasted for a few months after treatment ended. I did find that small naps helped.
I spent my lunch and my breaks sleeping in a chair with an alarm beside me. This would get me through the day, then when I got home I slept for two hours before I even considered a meal.
Slow but sure, my energy returned and I took less naps and started to get some physical energy back too.
Is there any more treatment planned for after radiation? Do you have the ability to slip in some cat naps?