The most important things you will take away from your 13 years of schooling do not relate to the HSC, something made painfully clear to my lot last year. You have already learnt so much, and gained so much, that will set you up for the rest of your life, simply about existing as a person. In terms of actual knowledge, the vast majority of the content of your HSC courses is irrelevant once you get to uni - particularly for you with the humanities subjects you are doing. As long as you can take away the broader concepts and the general attitudes/skills of the subject then you have got the point of it. The HSC then presents you with two opportunities - firstly a challenge, a goal to work towards (which is nice, but you set your own goals and that will determine how much stress you should have), and secondly a way to get into uni. You will surprise yourself with how well you do - the HSC is less difficult than they make it out to be.