For me, it would be A Level Further Maths. I lasted 4 months before dropping it, and those were 4 hard months… I didn’t understand a word the teacher was saying most of the time, which wasn’t helped by the fact that I was in a class with two people who were much better at Maths at me, meaning that I found it hard to keep up. I did two mock exams, got a U in both of them and eventually realised that it probably wasn’t to be.
An honourable mention would probably be the IOT Development class I briefly took at university last semester, although saying I took it would probably be defining “took” in the loosest possible sense. I thought it would be a module where we were programming on network simulators, but it was actual hardware and wiring, and I panicked after the lecturer gave me the huge wiring kit because when I started trying to play with it, I had absolutely no idea what to do… I may be a Computer Science student, but I am truly clueless when it comes to hardware! Suffice to say, I started trying to drop IOT the second I left that class (it was an optional module), and after spending a few days going through the process, I was eventually successful in moving to Theory of Computation!