Welcome to the Beyond Blue forums and congrats on having the courage to reach out! I hope you find this community a place where you can openly share your thoughts and support others! :)
I am in university right now, but I remember this feeling that you have described- knowing that you could do better but not preparing enough. Especially during the end of school- it seems like our grades are the be-all and end-all. Don't get me wrong, it is important to do as well as you can, but there are so many people who do not get the ATAR they want. When it comes down to it, doing a year of a similar course, then applying to get into your desired course will only delay you a year (nothing in the long-term). When you get to university, you will see people (even those that you think are really smart!) chop and change their degree halfway through! You will also meet people who are in their first year at 25! I have also changed a few times.
It sounds like you are really smart and will still do well. I completed an OP (a few years ago in QLD), so I can't confidently comment on your ATAR trial results- perhaps someone else can do it.
I have felt burnt out and similarly angry at myself in the past. The following are things that have helped me:
(1) Meditation/Mindfulness- focussing on the present
(2) Non-resistance/Self-Compassion- realising that it is okay to make mistakes, and that everything, in the end, will be okay
(3) Seeing a counsellor- usually free for students, but it is really nice to have a level-head to talk to
I hope some of this has helped! Feel free to reach out if you want to talk more :) Hopefully, some others can comment more on the ATAR side of things.