Bonsoir. I just saw your PM. For context, I got a grade 9 in Edexcel French back in 2018.
Have you had the chance to look back at your actual mock papers to see where you lost marks? Has your teacher given you any feedback regarding this?
You mentioned you are having some issues with grammar, I'd definitely recommend brushing up on those. Be specific and pick out which you are struggling with and find explanations/examples of them in practice. You can use a GCSE textbook for this as they have very good, concise explanations. You then need to practice writing these sentences out in order to memorise them and to get used to writing those sorts of structures.
Vocabulary is important. You said there are some that you don't know. Every time you see a word you don't know, and especially if it keeps cropping up in papers/exercises, you really should make note of it and its translation, and memorise it. You need to make consistent efforts to memorise your vocabulary, and as time draws closer to the exams, you should narrow your revision down to the words that you struggle with (instead of just recapping the easier terms that you know, which is counterproductive). Yes, Quizlet is definitely effective. Memrise is also a good one but I use Quizlet. See which you prefer. Both have apps that are downloadable on your phone and other devices, so you can revise both vocab and even grammar (e.g. verb conjugations according to tense) whilst travelling or in a café; wherever you feel like, really.
The CGP textbook is good. I didn't use it that much, but I did have one, and I remember that it was in a very concise format and gave good explanations of grammatical concepts. You can also find online exercises for GCSE French and past papers (Edexcel website) which you should find useful. Past papers are definitely important to do, but make sure you don't rush through all of them. You want to leave a few for when you are revising in Y11, a few months before the real exams.
When writing and speaking, do you have a vocab bank/list of certain phrases that you use? If so, could you please give a few examples. Sometimes these can be what are hindering your performance.
Do you find yourself struggling more in any particular areas? Reading/writing/listening/speaking? Try and be specific with what throws you off and I can try to tailor my advice a bit more.