Hi gracieconnory and welcome to the ATAR Notes forums!
In regards to your subjects, it's definitely possible to get the ATAR you need (assuming you have done a 6th subject that is).
Consistency is a crucial part of getting where you need to go, and just make sure that you are consistently doing work throughout the year.
Make sure you take note of any mistakes you make in those practice exams and make a note of them (e.g. literally in a notebook) and make sure to not make the same mistakes again.
Sine has beautifully said this already, but I highly recommend that you take the time to make a notebook/folder which contains makes you make throughout the year so you don't go making the same mistakes again at the end of year exams.
It's a great tool as it helps you see gaps in your learning and it gives you an insight of where you might need to revisit certain areas to study.
Also, about the 99.9 ATAR situation, are you currently taking a 3/4 sequence right now? Based on last year's scaling report, you needed an Aggregate of 207.39 to get that ATAR (it varies each year though but usually a small change). What this means is that your 4 best scores, including English and then 10% of your 5th and 6th lowest scoring subjects. So for example, an aggregate could be: (40) + (44) + (35) + (34) + (3.2) + (3.0) = 159.2 Aggregate (Let's pretend they were all scaled scores). I don't think that even with those subjects (assuming you only do 5 and haven't done a 6th) will get that aggregate? (Someone please correct me if I'm wrong). Most importantly, do you meet the prerequisites for your course?
Hope that helps!