Sounds normal to me! I spent years composing classical music hours every day, which was great fun, but after about 3 hours writing in the morning I'd be utterly, totally exhausted. Sometimes much later in the day I'd do more, but not usually. I've heard from various places that that's not uncommon, being worn out after a few hours of totally focused work.
I read somewhere recently that Michael Phelps (I think it was) swam for a few hours each morning then had a sleep, doing more later in the day. I never thought of doing that, not being a napper, although I like the idea of napping, but maybe I should've. I imagine relaxing by reading online or chatting or something doesn't have the same restorative effect napping does.
I get a lot of ideas when I go out the back and stare at the trees while having a smoke. Most of my good ideas. I think it's the sitting there, letting your mind do what it wants, absorbing whatever you were just working on. If that was a problem, often the solution just comes to mind. But that doesn't happen unless you have breaks like that.
Maybe you really like learning but need to pick up something else, to give your brain a break. Maybe you need to do something with your body instead of your mind. Burnout is real, and people need to change in order to grow. (We're close in age.)