Just chiiiillll
Keep reminding yourself that the HSC is a silly little exam designed to test your regurgitation abilities because it's too stupid to do anything else. Few people go into their exam thinking they're totally prepared, so don't stress yourself out too much
If possible, try to have fun with studying. I remember having a sheet about the 7 elements of Jacobean Tragedy up on my wall, and I would recite out those seven elements in this ridiculous accent which made me giggle
On a more serious note:
Assuming you're preparing for your english exams, practice essays are your best friend. Even if you don't think you know anything (this is so totally me two years ago when I was freaking out about Hamlet and R&G), just do one. You'll see at a glance what you need to work on, so look it up in your study notes. Then do another practice essay with the intent of polishing the stuff you do know, and integrating what you just learnt/remembered. Repeat process as neccessary until you are satisfied.
Aside from that, remember that this is your first "major" exam. It's new. It's big. It's frightening. Of course you're not going to feel totally prepared for it!
Once you get over this 'Boot Camp', all other exams you do will seem a bit more managable
