I had the same problem throughout the year, and my biggest fear leading up to the literature exam was that I wouldn't be able to physically WRITE two whole essay which I'm happy with in 2 hours. I mean, I can write well, but timing was an issue especially considering that I'd almost always typed my work on my laptop and spent hours changing the sentences around.
The only thing you really can do for this, and I know this has been said before, is to write practice essays in timed exam-like conditions. So, no laptops/typed work, no liquid paper, no music, no distractions, no nothing. If you can, have your teacher read over them and give feedback- I'm sure they'll be glad to mark extra work. I mean, in the end, your study score for English is reflective of not only your own efforts but also your teacher's.
Once you start writing , you really begin to see the improvement. You'll get into the habit of picking up on 'good' sentence structure and words that work well, etc and learn to reincorporate them into your work.
And be sure to have a detailed understanding of the text before you get to the practice essays stage. This way, you can tackle any prompt given to you and creating a plan isn't as difficult. Ideas will just flow (hopefully).
Also, even if you're not finished, leave at least 5 minutes towards the end to check your spelling and grammar. Often in exam conditions, you can end up making mistakes that you normally wouldn't make. In one of my earlier SACs, for example, I wrote 'ur' instead of 'your'. -.-
Also, since Year 12 hasn't actually started yet, I'd recommend typing up your work for the first term or two, and then gradually starting to write your essays on paper instead. Typing them out does help and really lets you organize your ideas; whereas, on paper, you often end up crossing things out and getting lost in the cross-outs.
Don't stress too much for the time being. You've got a whole year to prepare.