from someone whos just finished yr 12...dude- studying 7-8 hours a day in the holidays just isnt cool, no matter what you're aiming for. In the end, UAI is probably 1/5th talent, 1/5th hard work and 3/5th luck, because who knows whats going to come up on the day of your exam, what the markers will be like or how the scaling goes against you. As for stories of breakdowns, I had a pretty bad one in about April where I couldn't do anything except sleep- I'd get up after 14 hours sleep, then sleep all the way through class, and I'm usually a really dedicated student...the point is, I'm really glad it happened then and not before the HSC becuase it told me to lighten up. In the holidays before trials I went and lay on the beach in Brisbane doing nothing for a week, and did the same in Sydney for half of STUVAC. It meant that I was rested, relaxed, and ready to really hone in on what mattered when I did study for the rest of the time. My dream was to get the marks to do law at syd- so like you I was aiming high- and I did it! Not because I slaved away- the opposite- because I finally listened to how tired my body was getting and took time out. Good luck, but don't kill yourself- you will find no pleasure in living like a robot.
Cheers