I'm in my second year of A levels and I self-taught C1, C2, C3, M1, S1, D1 last year. I'm doing the same this year for another 6 modules. C3 and C4 are killers, there is no way around that, and M1 did truly make me want to cry. But there are ways of powering through (I managed to get above 90% in all, so my method does work).
If you have the textbooks, then start at the first chapter and work through every single question. It's time-consuming and repetitive but it definitely helps. If you're doing three modules, then try two exercises a day and it'll be done in about 9 weeks. (I'm doing Edexcel, and the textbooks for those are online if you need them)
If you think you've actually got more of the knowledge than that, and it's just plugging gaps and exam technique, then bring out the past papers. I'd heavily recommend that anyway, but the sooner you start the better. Do each one in as close to exam conditions as possible (although if you want to do them under a pile of blankets with a cup of tea in bed, feel free) and then, at the end, compare to the mark scheme. Google 'Physics and Maths Tutor' and you will find all the past papers you could possibly need, as well as model answers. Note down the things you commonly make mistakes on and then head back to the textbook - repetition is key.
Ultimately, there isn't much they can throw at you in an exam that they haven't already, so practise enough and you'll be fine!
M1 seemed like hell when I first started, but eventually it will click - trust me! The questions there are very repetitive and lucky for you there shouldn't be too many vectors on the exam (our paper last year was basically all vectors, which I reckon is the hardest). Master the diagrams and it makes a lot more sense.
I reckon the key to C3 is spotting the identities, it ends up in most of the questions. And again, the only way to spot them is to have done similar questions before (I know, you probably hate the idea of doing trig questions over and over again, but needs must). And know that arcsin means the inverse of sin etc - I didn't know this which really confused me when I was asked in the exam to sketch arcsin.