Felt the same but I think we were both wrong :D
"I used to be pretty school smart in subjects like math and comp sci" probably means that you felt smarter than some/many due to your surroundings (it's always in comparison with others).
That perception gradually is being corrected but it messes with your self image.
You weren't that smart, as in, imagine now trying to learn the easy things that learned at that time... you would still be great at it :)
The problems are harder, life is harder, your peers are better and it is all a bit less exciting because you have seen it before.
Thing to do:
* Improve your energy/capacity management. If you're tired, relax don't try to brute-force as it's usual when young. I sometimes half listening to a podcast, execute tests and read an article. Other times I low on energy/capacity I need silence, close HN and just focus on code.
* Find other things to excite you at your job... Maybe having junior devs be happy that you're helping them light a spark again.
* Do individual/team sports. Whatever allows you to clean your mind from work/tech and be excited to do it.
> Does anybody have a good way of testing their intellect or problem-solving periodically?
Join a code challenges site. You will find that you're pretty good at the easier levels :D And you can retest it regularly.
I sometimes used to a quick sleep deprivation test just to be sure that I was slow due to lack of sleep. I use to throw a pen in the air and catch it after 3 turns, it was very obvious the days where I needed sleep.