Lol, you remind me so much of myself in year 11.
I used to do so shit in tests because I never really studied/fully understood the concepts and the excuse I'd make to myself was that, 'Oh, it doesn't matter, the assessment throughout the year is only 40% but the end of year exam will be worth way more (60%), I'll just study and go well then.'
I'll give you an example.
My average throughout my physics tests across the year was about 40%.
I said to myself every time I received a test with a fail score, 'Fuck it, I didn't even study for this one, I'll study and go well at the end of the year.'
I did study but at the end of the year, I received a 72% on my exam, which was still pretty shit, considering it was a Unit 1/2 exam.
I was so lucky that I learnt my lesson in year 11, cause if that happened now, I'd actually be fucked.
In my opinion, if you can't go well on your SACs, you'll struggle to go well on the end of year exams.
You have to start studying thoroughly and ensuring you understand everything NOW, and never say you'll do something tomorrow, cause I can assure you, tomorrow you'll repeat what you said.
If I were you, I'd go see my teacher and ask him/her to guide me through what I need to do, what I have to do to learn from my mistakes and most ultimately, how can I improve.