I'm going to be resitting 8 gcses and maths aswell, I got a grade worse than you did and was 2 marks of a 5. I did some research and spoke to my career advisor and they told me that in a span of 8 months, I'd need to do 8 hours of revision for 8 gcse's a day. So if you can fit in a minimum 90-120 minutes a day since there are only 5 months left till gcse's begin then you should be able to. If you revised for 90 minutes per day then you would have revised for a total of 231 hours which is good considering you wouldn't learn guided maths in school for more than 80 total hours across the 39 week year. You could do even less than this depending on how fast you learn. My doctor has also advised me to drink water to improve memory and lessen brain fog. Memory is a chemical process, memories can be triggered by cues which is why people say you should chew gum when you revise and chew it again during your exam. Pills like lions mane or pills including ingredients such as: magneisum l threonate, bacopa monneri, ginko biloba and l theanine are amazing supplements which help the brain to function in different ways, I personally use FocusFusion by Troo Healthcare which contains all of these and more, it's 13 pound and provides you with 1 month's usage, if you want to use less money then just opt for lions mane with fruiting bodies. (You can get it off amazon.) Anyways, hope you do well.