Sarah, the same thing happened to me in Year 12 sweetie.
I was dux of Year 11, would've easily gotten a UAI of 95+ and was so motivated.
I became really burnt out after the first Term of last year, just after my mid-course exams. I couldn't be bothered doing anything related to schoolwork and my marks dropped dramatically between mid-course and trials.
I didn't do anywhere near as much work as you've been but it wasn't necessary for me, I had a natural ability in my subjects. At the end of the day, I got a UAI of 81.40, which isn't fantastic but it was enough for what I wanted to do.
My advice? Don't stress, take a few deeps breaths (trust me, it helps) and just do the best you can. These exams don't count for much in your overall HSC mark so don't worry if your marks aren't as good as you'd like them to be. There's so much time between mid-course and trial exams for you to get back into your study rhythm.
Like others have suggested, try the timetable and rewards idea. It worked for me when I decided I actually gave a shit about my HSC.
If you need anymore help, feel free to PM me, even if I am at work.