I recently had, for various reasons, improve my math skills.

I was surprised at how difficult I found math. Now, I was never really great at math; logic and calculation in the head I could do fairly well (above average), but just foundational knowledge was hard and mathematical theory even harder. But now I even had trouble with integration and differentiation and even with understanding a problem to put it down into a formula. I am far from being the youngest anymore, but I was surprised at how shockingly bad I have become in the last some +25 years. So I decided to change this in the coming months. I think in a way computers actually made our brains worse; many problems can be auto-solved (python numpy, sympy etc...) and the computers work better than hand-held calculators, but math is actually surprisingly difficult without a computer. (Here I also include algorithms by the way, or rather, the theory behind algorithms. And of course I also forgot a lot of the mathematical notation - somehow programming is a lot easier than higher math.)