sanner I always had a similar experience, I imagine it's probably a mix of a few things.
-Lack of interest/boredom
-having to sit still for long periods
-having to try and focus a lot is quite tiring.
Forcing yourself to do a task rather than naturally being motivated to adds like an extra 40% to how tired you get imo
I also spent a lot of time not sleeping when i was supposed to which probably had a big impact overall too