Like I said on a thread a few months ago, the three week period that you do your exams goes way faster then any three week period in the holidays. Using an ATAR calculator will give you a really good idea as to what you will get (they are usually correct to within 1.5 ATAR points, for me they were correct to 0.3-0.4 points). If you do poorly, then your internal ranks can only save you if your year does well. If they dont, the internal ranks probably wont do much to save you.
Look, if you worked your best for the HSC, then there is nothing else you can do. I told my parents after my last exam that it was my best effort (which is true), and whatever my results were thats the best I could do. So go over your efforts and be realistic (you can only do so much) - if they are your best then there is nothing else you could really have done.