Compared to who I am now, I was very stressed in Year 12. For the most part, I was able to harness this stress and use it to drive me (okay, I didn't exactly do a lot of practice exams and such, but still... I studied!).
Towards the end though, before exams, I ended up crying a lot. I think it was a bit of exhaustion building up as well... I found it difficult to sleep in the days approaching exams. I couldn't sleep because I was stressed, and I was stressed because I couldn't sleep. I had about 3 hours sleep before my physics and accounting exams because of this issue.
Point is, you guys are not the first, nor will you be the last, and you are most certainly not alone

Try to find something that will help you de-stress a bit... when I found myself unable to sleep, I found a deep-breathing exercise helped pull my heart rate down.
At this point, people finding themselves stressed because of the pressure should head back to basics... make sure you're eating properly, getting regular exercise, socialising (as in, don't drop off the face of the planet, but don't go overboard on this either), and giving yourself at least half an hour between when you finish studying and when you plan to go to bed (read a novel or watch an episode of your favourite show or something, so long as it allows you to just switch your brain off and enjoy it).
Trust in your ability to pull through - You CAN do this guys! ^_^
You've all put it far too much effort to give it all up now!!