For people who are getting Ds who want to be getting > C, the issue usually is lack of understanding rather than poor exam technique. A lot of teachers simply keep teaching assuming you already know everything you have ever been taught. What this means is that a lot of people end up having to go through GCSEs learning complex things without getting the fundamentals. For each subject, see if there's some kind of pattern of stuff that you really struggle with, it could be a clue to something that doesn't click for you. See if your teachers can help you with this.
My advice would be not to revise too too much. It has diminishing returns, especially if you are struggling with the topics in the first place.
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