This is good; it is important to identify where you went wrong last time to avoid it happening again. You say you did all that study and still weren't able to reach your goal? If that is the case, it is likely that some bad luck on the day, coupled with your illness dragged you down. If such a thing were to occur again, I suggest not hesitating in getting an Illness/Misadventure Form.
On the other hand, was it perhaps the quality of the services offered at your school and/or an inexperienced teacher? My honest advice in this instance would be to find an Ancient History tutor, even a past HSC student from recent years to help you with your work. I know they're very difficult to come by and may be expensive but trust me, if they're good it will be well worth it.
My situation was a bit similar to yours except that it was with Business Studies. I was good at it, but was still falling short of the high goals I set myself. Some people think that getting tutoring when you're already doing well is a waste but I strongly disagree. In my case, it helped me improve my understanding significantly and always kept the subject ticking in the back of my mind which certainly led to a better mark for me in the HSC.
Finally, it may be that you've set yourself a goal that is just too high. An ATAR of 98.5 is no joke; only a few thousand people achieve or exceed it, if that. Is this a personal goal you've set for yourself or a specific ATAR you need to achieve in order to get into your preferred course?
I say again, until you localise the reasons for falling short, redoing the course is pointless IMHO. Once this is done however, I have no doubt that your determination and drive will see to the rest.