It is not enough to know and understand all individual topics. You need to be able to analyse a question and decide upon which strategy from all the strategies you have learned, will help you to find the correct solution. For especially on the Paper 4 question paper it will perhaps not be immediately clear which mathematical topic is being tested. But be aware that questions will never be about things you don't know or didn't learn. The whole point of the paper is for you to bring all your knowledge and skills together and discover what is needed to solve the question.
It is also still extremely important to read a question carefully and decide upon how to approach it rather than just diving into it. Think about what answer you would expect (eg if you are clearly calculating an acute angle, then every answer above 90 will be wrong. If you are calculating probability, your answer should be between 0 and 1). When finished, check whether your answer makes sense and whether it is roughly what you expected it to be. Common sense here will bring you very far! Rounding also remains to be a problem. Please take satisfaction and pride out of your workings and answer and treat it with respect. That means – don’t just jot it down, but round it properly and look at it for a while.
Finally, your GCSE IGCSE maths exam is one big puzzle. Do not try to overcomplicate things but challenge yourself to find a way to tackle problems and I am sure you will enjoy it a lot more and become more successfull!