Everyone really hit the nail on the head.
Just for further clarification on the SACs < Exam thing - the reason for this is moderation.
When you take the standardised exam, your exam score will be scaled/moderated according to it. Because all schools have SACs of varying difficulties for the same subjects, they have to use a standardised test to level out the playing field. Thanks to this, if you do well on the exam, your moderated SAC mark will also reflect this. Of course, the reverse is also true.
The one piece I can confidently give you as advice (being a Rank 2 who got a study score higher than Rank 1, also):
Keep in the present. Thinking about your SAC will only demotivate you. You can't do anything about it except for channelling your emotion into something more productive. Something I also see mildly reflected in your post is perhaps looking too much into the future. It is just as detrimental to be too comfortable with your current standard of work as it is to be terrified by the thought of exams, how every tiny thing will affect your ATAR/what your ATAR 'means'. Keep in mind that you're against not just your school, but the state. Aim higher and higher and aim to excel not at your class's standard but your own and the state's standard.
I don't know if that will help you - but I'm sure you'll find your way. Good luck.