I finished my HSC last year, and I understand the stress. I understand the cycle of not getting work done because of stress, then the stress getting worse because it piles up and you aren't sure where to begin.
I think the most important focus for you right now is self care and your mental health. You mentioned that you aren't sleeping at normal times or eating, and that you've been feeling overwhelmed and depressed. No one can expect you to perform and get things done when you're going through this, so please be patient with yourself as best as possible.. This is especially challenging during lockdown, and I'm not sure if you're a year 12 student doing everything from home, but that is incredibly tough.
Please make the small steps to look after yourself and your mental health, whether it be reminding yourself to eat every few hours and drink enough water, going for a walk, doing the things you enjoy. If you're comfortable, how would you feel about reaching out to one of your teachers to talk about how you're feeling?
As you said, you still have time to prepare for your exams. Start small and write some to do lists for each subject so you have a plan for what you need to do, and cross things off day by day. Taking the time to actually know what you need to get done is super helpful, even more so than forcing yourself to sit down and do the actual work.
Doing year 12 from home for me left me very overwhelmed and depressed, to the point where I nearly gave up. If you don't get the marks you need for the course you want, that's okay. There will always be a way to get to where you need to be, believe me. Even if it's not the typical route or the one that you expect. As I mentioned, the expectation to get a 96 atar while doing everything on your own is insane, all the while experiencing depression, isolation, all the things that come with lockdown.
I relied on early offers to get into the course I wanted and SRS, but I prepared myself by making a backup plan if I didn't get the marks I wanted (ie. what TAFE courses or pathways I could do to get into uni). It brought me a lot of peace of mind so I didn't feel my marks were the "be all and end all", and I found I could study more easily without that intense pressure.
Year 12 is so hard. Please be proud of yourself for making it this far. You're doing the best you can for right now, and that's okay. Be patient with yourself and take care of yourself, that's the most important thing.