First of all, I would like to ask you a questions.
Do you want to do Chinese? Why/Why Not? Please give an honest answers to this. If you don't want to do Chinese, then quit. Its not worth doing a subject you hate for VCE as unit 3/4 since you actually want to be willing to work hard on the subject.
You have mentioned limited vocab in your post. During your studies, you should have been exposed to a variety of sentence structures other than the one you mentioned. Last year, when I was doing unit 3/4, I was exposed to sentence structures such as the 如果 _______就 (If... then...); 虽然______但是 (Although...But (and please don't translate sentences with this sentence structure. Just use one of "although" or "but"); 因为______所以 (Because... So (again, don't translate it like that. Use either Because or so).
Perhaps, if you have been exposed to them, you could try using them in your essays. For example: if the topic was on 学生打工的利与弊, you could argue that 因为学生有了工作经验,所以他们的工作机会就提高了。(Don't write this as topic sentence in your essays) Practice using these sentence structures during your studies. You have another year. You should be able to increase you essay mark. I went from writing essays of a C+ grade to an A within that timeframe (but stuffed the exam), so you still have time, given that you put in the effort to improve your essays.
Honestly, I am a mid-range student in Chinese Second language (since Chinese is a competitive cohort), and this may not be enough for SLA. Instead, you may need further consultation. I suggest you get your teacher to help you, or to get a tutor.