Don’t overthink it! I’ve just started year 12 too, I’m doing the same subjects as you plus geography and I actually got an E in maths! I was definitely disappointed, but you have to realise that there’s so much time to improve, and I can assure you that you’re not the only person in this situation. It’s a huge jump and it’s very overwhelming. Keep studying, I’m sure teachers will be more than happy to go through topics you’re unsure about, they’re there to make sure you do the best you can after all. Sounds cheesy but don’t give up, keep working at it and you’ll do really well.