Trigonometry. My high school math teacher was a literal math genius and would always go deep into proofs and theory, sometimes not even getting to our homework stuff until the last 5 minutes of our 50 minute class. As a result I went from the "gifted" math group to nearly failing.
When I went to college I had to take a math placement test and ended up in Math 99 (below college level math).
It was there I was finally taught SohCahToa and everything clicked. I actually use simple trig a lot in my job now.