I have a son similar age as you who is going through the exact same thing. He's always been more advanced than his peers academically. He gets a sense of both identity and satisfaction from being top of the class. The burden on you "smart kids" can be huge and it is very important that you learn to manage the pressure. Life is a long journey and it's just the beginning for you. It's like running a marathon and you're trying to be in first-place all the way from start to finish.
I can imagine most of the pressure is coming from yourself, right?
If you look at the bigger picture, this is a never-ending race. Once you've achieved the score for your first-preference uni course, your next challenge is to continue being the smartest student who wins the faculty-medal. After graduation, will you get the top graduate job? Once you've settled into your dream job, you think about how to get noticed and be promoted quickly on the corporate-ladder. Then comes the high school reunion, have you achieved enough in life to show-off to the others? Or will the others think you had already reached your peak in high school?
Maybe you can try to adjust your thinking. Instead of focusing on the 99+ ATAR, are you able to focus on just trying your very best? If you've already made your best attempt, there's nothing more you could have done.
Here are some of the things that I tell my son. I hope you can find something useful from the list:
If you look at the many years of your life still to come, you would realise Year 12 & the uni years form just a very small part of your journey.
Study your hardest and prepare for every exam as best as you can BUT do not sacrifice your sleep.
You need a healthy balance in life. "All work and no play" is counter-productive. Set aside leisure time every weekend and stay away from your books/computer. For example, watch a movie every Saturday evening after dinner.
Medicine and law are NOT the only career choices for the elite. For your generation, there will be a wide range of new professions/career opportunities. (eg. robotics)
Grades only reflect how well you do in the short-term during your schooling-years. People-skills are far more important in the long-term for your career (and every other aspect in life).
I hope this can help you!