Congratulations on graduating!
I think that part of the problem may be your perception of graduation. While graduation is definitely a wonderful milestone, your HSC journey is not over. You should treat graduation as an opportunity to reflect on the progress that you have made throughout year 12, but more importantly as a reminder of the fact that you are approaching the HSC exam period and must do your utmost to prepare for your HSC exams. In helping you do so, you should constantly remind yourself of your goals (which are usually the biggest motivator) and how important it is for you to achieve them. Think about the fact that you do not want to regret not thoroughly preparing after you finish your HSC exams, and that you do not want to wish you could go back in time and do things differently, as this is definitely not a nice feeling.
Once you recover your motivation and your determined mindset, you can start preparing for your HSC exams. It would be a good idea to design a study plan to provide yourself with guidance regarding the way to approach your preparations for each of your subjects. It can also include expectations and reasonable goals, such as the subject(s) that you would like to prepare for on particular days and the rate at which you would ideally want to be making progress.
I hope this helps!