Hey,
I was in your position last year. I had disappointing SAC marks, and at my worse point, I went three weeks straight without studying. I lost all motivation until a few weeks before my exams, where I studied my ass off, and managed to gain a decent score.
I found that the longer I put off starting again, the worse it got. The more time I had off school, the more anxious I felt about returning. It's a bit like pulling off a band-aid -- you do it fast, rather than slow. Rather than waiting for another day, week, or even month to start something, you've got to do something right now, even if it is something small, like reading through your notes or laying out your uniform for tomorrow.
You don't have to tell anybody your SAC mark. Don't feel obligated to.
Your ATAR IS salvageable. Your exams count for a lot. Additionally, there are always other avenues into your chosen pathway -- you can always find a way in.
Remember, ATAR Notes is always here to support you or help you with concepts in your subjects. Your teachers are there for you too -- reach out to them.
The important thing is to start as soon as possible.