I was in a situation almost the same as you a few months ago. I was taking 3U Eng + Math, Chem, Phys, Latin and SDD. Naturally I found that too much and nearly killed myself by trying to do an AMus and go on a music tour with my school. I immediately came back and dropped Physics, because I didn't like the course in year 12 and couldn't understand any of the courses, so I felt I ought to drop the subject I was sure would be binned for the HSC.
I suppose all I can say is, drop the subject you a) won't do next year or b) do crappily at or c) can't stand, but if you're as extracurricular-ed as I think you are, something ought to go.