Warm welcome to the BB Forums :) Sounds like you just made a good choice in reaching out here, a brave choice in that. I say this because I know for sure you are not alone in feeling this way, and it is actually quite common experiences for those sitting their HSC, and a common thread to appear on the forums. This is a good thing - by making this post you have not only allowed yourself an opportunity to seek support and feel connected, but a sense of ease and connect for others reading who may be feeling similar to you. I believe it's so common its actually been names "test anxiety", or "exam anxiety". Know that just because it's common doesn't not mean that its any less crap, to put it lightly. Your experience is unique, unlike anyone else. Jasjit question about what your anxiety expresses itself as can help us understand yours, so please share if you feel comfortable doing so.
From what you have said so far, your personal experience of exam anxiety do sound super debilitating, to the point where you cannot possibly preform at the level you know and want to. This must be incredibly frustrating.
I'm sorry that you are feeling alone in this right now, and that no one seems to really understand at the moment. For me personally I sought understanding in talking with my teacher. This was scary and honestly felt stupid and pretty childish but it was nice to hear from someone with so much more life experience and knowledge. Is this something you have tried or thought about? Sounds like a listening ear and some advice from lived experience is really what you need right now.
These times are super crazy and stressful - especially to be going through HSC. We really feel for you and are cheering you on from the sidelines. Please keep us posted with how you are getting on <3