My HSC was a complete disaster, and I knew it about 6-12 months before the exams.....just wasnt interested in all the study. Then 18 years later started Uni and nailed every subject, quite a few HDs. My salary really jumped as a result of some qualifications I got under my belt and a couple of strategic career moves.
Lesson being, a poor HSC result ain't the end of the world. Now get back to the books!