You should ask for evidence that you plagiarised. An accusation of malpractice is serious because malpractice is serious, and it should be substantiated. At this stage, you should learn what you wrote and understand it, so should you be asked again by someone else on what you meant, you can actually explain it.
But you've shown no display of your own understanding, which makes your teacher's suspicions valid. You are rote learning, using prescribed quotes, didn't read the book. Maybe you shouldn't fail, but you shouldn't think you deserve a great mark as well.
The Board of Studies has stated that: "Should malpractice be suspected, students will be required to demonstrate that all unacknowledged work is entirely their own."
You have not shown that to your teacher, so the conclusion, whether it's correct or not, is that you have indeed plagiarised.
If you actually want to finish the HSC, why would you suggest giving up? Why is anyone else's opinion relevant in your decision on finishing the HSC?