OP - you just need to practice papers. Wasting time on one Q does nothing to help you.
It's easier to gain average marks doing all Qs, than gain perfect marks doing one, and then failing the others by running out of time. Think of it as an increasing curve, the difficulty in gaining full marks (for essays in particular) increase drastically - thus your time is best spent ensuring ALL your Qs are done.
Figure out, if possible, how much time you have to do each Q and stick to it. Plan each at the start to save time.
All this applies when you get to uni to if you go.