Don't take bad marks to heart. But this doesn't mean you should completely disregard it! By all means learn from your mistakes, and use it as motivation to try harder, improve and achieve your personal best. And like what has already been said, one mark will NOT determine your final end result. And I believe this because this is what happened to me in Year 11 (I started out with average marks around 60% and ended up ranking first in my cohort). And plus the good thing is that for most subjects, the first assessment is weighted the lowest and teachers usually take into account that this was your FIRST EVER HSC assessment task. I truely believe there is still so much time for you to improve, keeping persisting and eventually, come out on top!
It takes time to move on from a bad mark, even I struggle sometimes. So just acknowledge the mistakes and the marks lost, and use this as a drive to improve for the better! The HSC is one heck of a rollercoaster but you will eventually reach a stop and you won't have to worry about it all after its done.