Damn, that sucks. There's actually not much you
can do outside of extra work, extra help and feedback. Do you know what your problems are, specifically? If you're doing all the questions, maybe your problem isn't so much with the skills related to the content but test anxiety or poor teacher (not money, silly. Like if the teaching isn't an issue, maybe they're just bad at writing assessments. Hopefully neither, but getting stuck with both would be pretty awful). Are you doing the extra study all at once, or staggering it across your average week? That can make a pretty big difference in the long run, too.
It's really hard to pinpoint what would bump your grades up if I don't know what's knocking them down. Have a look at these questions and let me know which areas are giving you trouble (you can PM me, if you'd prefer

) and I'll try and give you some more specific advice.
Spoiler
Are you confident with the content?
Does the work require knowledge from previous years that you're not sure on? (Like BIDMAS, teacher assuming you know x or y formula that's mostly irrelevant, bearings)
Are you getting significantly different marks between calculator and non-calculator SACs on the same topics? Why?
Do tests make you nervous? (apparently they shouldn't? #RIP)
Are you having trouble with certain types of questions? (like worded questions?)
Are you making silly mistakes? (whether you're confident or not. These can turn a C to a D or an E if you're not careful)
Is your work written out clearly?
Ideally, the answers to these should be:
Yes, no, no, no, no, no, and yes.