Hey, people who are doing badly in mathematics won't be much good at University. Your choices at University will be very limited. That's not even talking about the fact that you virtually need math for everything nowadays. Even Biologists now need hard math as the science is moving into molecular level! Social science requires computing and math as well.
Why do some people do so badly in mathematics? I belive that any one who is not naturally good at mathematics should be able to be reasonable at it if the training is right and enough effort is made. The reason you are lousy at HSC math is because you are lousy at primary school math. Students who are lousy at HSC math won't be able to deal with difficult math problems for grade 5-6. Yes, even if you drop back to grade 6 and try hard stuff for grade 6 you will also fail.
The solution is simple but it takes time. The only way is to go back and learn lower grades. In many cases I would go as far as suggesting that year 11 students go back to grade 1 and work rapidly forward. Spend an average of 1 week on each grade from 1-4, then 4 weeks on grade 5-7, 8 weeks on grade 8-9, ... This depends on individual ability. Students who do this will start to discover all sorts of gaps in learning that they did not cover when they were young. Only by filling these gaps then restructure one's understanding of mathematical senses (number sense, fraction sense, ratio sense, geometrical sense, algebraic sense ...) in a solid manner one would be able to move forward. What they generally do when they have lost the basics is to rote-learn. And that's terrible. They will forget almost everything 12 months after graduation!
I studied mathematics at University level and I still learnt a lot by going back to grade 1-6 to see how the teachings have changed over the years. I still learn new things and new ways to solve problems!