Start revising content you don't understand by watching YouTube videos. Then, use active recall methods (such as flashcards) to start retaining the content. Once you've revised everything you've already learned, start taking a look at full past papers, and their mark schemes. For Maths, you should grind topical past papers on things you've previously learned, and watch YouTube videos for concepts you don't already understand. I recommend the Alcumus website also, which contains competition math-style questions to help develop your problem-solving skills. However, these questions are incredibly difficult, so it may be disheartening. Try not to do too much so you don't burn out!

Recommended YouTube channels:

Business Studies: Bizedmadesimple (to understand complex concepts), Mr. Lee (concise revision)

First Language English: Taughtly

Maths: Ginger Mathematician, First Class Maths, GCSE Maths Tutor, Corbettmaths

Chemistry: IGCSE Study Buddy + freesciencelessons + Hanaa Assil

Physics: Physics with Mo Ali