Year 11 is a huge change for everyone and it is understandable you're finding it hard to adjust. I suggest you give your best shot in year 11, working as hard as you can. Year 11 is a practice run for year 12 and it's a good time to test out what works for you (e.g. Managing workload, Study techniques). If you slack off and only do bare minimum you are not preparing yourself well for year 12 and by the time you start the HSC you won't have any motivation.
Even though the Prelim content wont be tested in the HSC some concepts carry over and are still relevent throughout year 12.
On the other hand, it is important to have a balance... such as allowing adequate time for social, work, exercise and relexation. I can understand how stressed you are. I am stressed the majority of the time. There are people out there that can help you, such as your year advisor, school counsellor and the community here on BOS.
Good luck and remember year 11 isn't the "be all, end all"