strawberrye and rumbleroar are on the absolute money as always.
I think it's really important to recognise what is a need, what is high priority and low priority. If you have something due in two days, you need to be working on it. Something due in a week is high priority. For a break, wasting the time on reddit won't be as refreshing as going for a half hour walk with the dogs etc. It's with that mindset you can maximise time and minimise stress.
Have weekly goals (or monthly goals depending on frame) to have certain things done. A goal like come 1st-3rd in everything at prelim exams will be less likely to work if you don't break it down into achievable snatches. And then ticking things of you get a win down, and if you have anything like my personality, I work extremely well of positivity, and negative motivation (fear or a bad result) only hinders me further the next time around.
If you don't meet a goal, be constructively kind to yourself to find one thing you could do better next time like say you only got a C on your story-seek a second opinion on your creative or practice writing creatives off stimuli etc. Don't negative self talk and be like "OMG I suck at writing etc. I'mma get a band 3 etc.". Only constructive to move forward better for the next task.
Some people are big timetable people, but for someone like you with a seasonal sport or comp, there are times when you will have more EC commitment, that's why I'd think to do lists would work better for you. Weeks when I didn't have a comp I would have like an extra 5 hrs to play with and stuff, so yeah, for people with a variable lifestyle I find they work best
yr 11 is the time to experiment with study habits, and also getting a feel for how many commitments to take on in yr 12