happens to me as well... you feel so down, you feel like giving up. but the thing is, you shouldn't... the hsc will be the last exam, the last chance, you might as well give it al you have got... the last thing you want is to look back at your hsc results and think to yourself, "shite, if only i have studied..."
the tips about basing your study notes on the syllabus is good idea, its what did, and my trial position for chem went from 14 to 8. it might not sound that good... but at least, i improved...