Three hours of writing? Damn you're killing it, I usually do zero
Jokes aside, I think this is normal. I'm assuming you're not literally doing nothing but worry for those 13 hours, which like, yeah, you're pretty much spot on, you need to let your writing ability recharge, and luckily you can do that asynchronously with other mental tasks.
Think of it like, in a video game where you have multiple weapons with individual cooldowns, but you can let one cool down or regenerate ammo or something like that while it's sitting on your belt, and you're actively using a different one. The brain is like that too.
But again, if you're actually getting three hours of writing on paper a day, that's genuinely super good. You seem like you're doing good to me in general. I don't say that to diminish your stress, I say that to compliment you and reenforce that you seem like you're on the right path, at least assuming I interpreted the actual situation from your post correctly.