When I joined a very demanding part of the French education system [1], my grades started to drown. I had basically never had to study to be top of my class in High School, and suddenly I was in the bottom quarter. I resolved to work much harder, and my grades went back up, but by the half of the first year I was still not where I wanted to be and I started feeling depressed and wondering what I was doing there.
At the time, there was one national exam you could take at the end of first year to join a mid-tier "École" (engineering school) directly. I knew I was good enough that I would almost certainly pass, and I decided to take it. There were two days of written tests. The first one went well, but as I was walking up to the 2nd day of tests I was mugged in the street. I ended up spending that day in a hospital, so I couldn't take the test. Now I had no other choice but to stay in CPGE for one more year.
After that episode I decided I couldn't continue like before, something had to change.
One of the main things I changed was that I stopped transcribing classes. That was considered heresy by teachers, who told me I would certainly fail. You have to understand that math teachers (not all of them, but mine did) wrote their lecture on a black board, and you were supposed to transcribe all of it. You would end up with binders full of math demonstrations that you were supposed to learn by heart.
The problem is, I am completely unable to focus on two things at once, and writing takes focus. So while I was copying what was written on that black board, I was not understanding it. Just the math classes took something like 16 or 18 hours per week, and I was wasting all that time. So instead, I bought books. I would read them in advance so when I went to class I knew what the teacher would talk about, and I would understand the details and the demonstrations.
The result? My grades went back up. At the competitive exams at the end of the year, I got the best rankings of my class, better than those of students who had taken the 2nd year twice.
So of course, this is anecdote, but I just wanted to say that what you wrote resonates with me 100 %.
[1] https://en.wikipedia.org/wiki/Classe_pr%C3%A9paratoire_aux_g...