Hey Pakisman
It's really easy to be overwhelmed with the overwhelming expectations of not only your parents, but generally your social sphere (as you attend a selective school). Burning out (during the HSC) has always been something that was creeping at the back of my mind, but gladly I never burnt out (well, not to a significant extent anyway). I guess it was because I always had a goal in mind (e.g. I really wanted to do Speech Path at USYD) and that was the little motivational push I needed to get through the HSC. If you set goals influenced by other people (e.g. your parents, your friends), then most likely the drive to achieve them will be based more on pleasing them rather than yourself, hence having a snowball effect because you are anxious to do so, etc.
I guess that first thing you can do now that you are aware of the marks you can achieve is to set yourself a realistic goal that you can work hard towards as the final months of high school approaches.
Pretty much the posts above cover the other advice I would give you (like taking breaks, taking time off, ranting and talking to friends, etc.), but the final piece of advice I can give you is to reduce your stress levels. Honestly, that was my biggest enemy during the HSC. Stressing about everything will do you no good. All of my friends told me that I should relax, but I just kept panicking and over-studying (I remember I re-wrote all my notes for Business Studies after trials... at the end it didn't really help lol). Trust me, pace yourself and just know your content. No more, no less, you'll do fantastic
Good luck and I wish you well with your endeavours with the Higher School Certificate.