Hey there!
A few points;
a.) I'm a VCE'er so the perspective might be a little different.
b.) I haven't personally experienced similar mental health difficulties, so again, perspective might be different.
That's said...
I also felt like I performed very mediocre this year, particularly in my English exam. Being my first exam, this put me in a bad mental frame for further revision for following exams. After exams were done I spent a lot of time thinking over what went wrong, and throwing what-ifs out there. But the major thing I realised from it was that beating myself up for past errors didn't change them.
I had pinned most of my failures on English (which is unfair of me, to say the least), so over the past few days I have been learning German. I guess it's kind of a denial strategy thing, basically “I stuffed this language so I'm gonna learn this other one...”
Hopefully this will tide me over until ATAR release, which will either confirm or deny my Mediocrity.
Tl;Dr? I know we just finished 13 yrs of learning, but voluntarily indulging in something that you are interested in can be a way of helping with post-HSC deflation, along with a multitude of chillaxing that comes with having finished school...

Hope this helps, but if it doesn't, please be free to tell me so!

Edit; I just realised that I hardly answered your question, but I don't know exactly how to answer it rn..
