Don't get me wrong, your ENTER is important if you know straight away what you want to do. But I got into a course with limited places, decided I hated it and dropped out.
One year later, and I may not get into a nursing course, although my ENTER was almost 30 points higher than the required ENTER for year 12 students.
You should always strive to achieve your best, but don't beat yourself up too much if you don't finish with 99.95. Even if you don't get into the course you want, there are always ways to work around it.
Just enjoy year 12, because you will miss school when it's finished.