I think you should breathe..
Calm down and look at this situation objectively without freaking out.
Your in Year 11.
You do subjects which don't scale as well as some that are offered, but if you go well in those subjects, as opposed to going bad in harder subjects, you will get a good UAI.
Highly doubtful that they will let you do Extension 2 english because you haven't done the preliminary course, but its not impossible.
There was a girl at our school who did standard english for all of year 11, and realised she's made a big mistake, and applied to the BOS to move to Advanced, and now she's doing advanced.
Be wary, though. Extension English isn't as easy as people seem to think. Extension 1 english is an interesting course, but having a major work hanging over your head as well as your Visual art major could make your marks in other subjects suffer. And, comparably, Extension 2 english doesn't scale that well. But, if you were able to get into ext 2, then you would be able to drop two other subjects which will allow you to progress with 10 units and less work, giving you the opportunity to excel in the subjects you keep.
Bearing in mind that there are a lot of options. If you truly believe that you aren't going to do well, then apply for early entry to a regional university, and participate in lots of school activities. They look good on a CV, and will assist you to get into a uni course. Then, after you have successfully gotten into a uni, transfer to the one you really want to go to after a year if your want to.
But, for now, concentrate on study and keeping up to date with assessments.
Honestly, I think you will be fine, because your freaking about your marks now, which is a great deal better than alot of people who dont care.