GCSE Mocks Advice

Different revision types work for different people, but if you're not sure what works for you, I would recommend trying seneca. You pick a topic, read the info, then do the questions on it. Start with the areas that you know you struggle with the most and slowly work your way through. Just small amounts are better than nothing, and hopefully, once you get going, it will be easier.
If starting is your issue, I have 2 suggestions for you: the 10 minute rule and a pomodoro timer. The 10 minute rule is as simple as it sounds: tell yourself you will do 10 minutes of a task you don't want to do, and you can move on to a different task after 10 minutes if you hate it. Most of the time, you will be fine to keep going once you get that first bit done. A pomodoro timer is a study timer in little steps, for example 25 minute study then 5 minutes to get up and move around, then 25 minutes study etc. After 4 study sessions, have a 10-15 minute break and then continue. I find this really helps me get going.
Hope this helps!

I already use Seneca (i love it so much. legit my only form of reviosn ngl) but will 100% try the 10min rule!
I tried to use the Pomodoro last year and wored like 75% of the time
😅

This is a method I used specifically for science subjects during GCSE which helped me a lot. First I would pick an area of content to revise and read the CGP Revision guide book or watch free science lessons video whilst taking notes. Once I felt confident I understood my notes, then I would blurt everything I remembered onto a whiteboard helping me identify the gaps I needed more work on. Then ,this is the most important part which I would spend 70% of my time doing, I would practice exam questions on that topic from Physicsandmathstutor.com. For reference, I went from 4,5,5 in triple science to 8,7,7.
For maths, I loved watching the gcse maths tutor on youtube as he would explain concepts and do exam questions walkthroughs on that topic after. I practiced a lot of exam questions off physicsandmathstutor.com. This helped me secure a 7 in maths.
For humanities subjects, I don't remember exactly how I revised, I was quite good at it naturally so I just found it easier, but I did quite a bit of blurting on a whiteboard for memorising content and I had set solid structures for answering questions which I would always follow. I got all 9s in my humanities subjects.