Oh, I have quite a few big mistakes.
1) Not working for UMEP semester 1. I did a bit of hubris and took on the attitude that because I understood the material, that I didn't need to study. Big mistake.
2) Completely bludging academically throughout semester 1.
3) Not doing enough violin practice.
4) Not doing specialist maths or literature... or biology. Gosh, I would've done so much better in those subjects.