I think most people have answered this one, but for what it's worth, definitely not too late. The content of a GCSE module spec is not an unmanageable. Generally for people who have worked consistently over the course, very little 'revision' is needed (i'm not saying little knowledge/effort/etc. before I get shot lol) since they'll know the concepts already.
In your situation, going by your mock grades and confidence levels, the problem is your exam performance NOT the amount of time you have or haven't spent revising already. Trust me, get that hindsight I should have started months ago feeling of regret out of your head NOW. You'll get it at every stage in academia, and it will never be helpful beyond getting your to pull your **** together on that actual day itself. I know people in A levels who spend all day every day in the library and still cannot perform in exams beyond a C/D. Efficiency of your revision is key - not the amount of time.
Some practical advice:
You've been going to revision sessions for maths, this is helpful providing you used them to figure out where you're going wrong with answering Qs. In any case, this should relieve some of your guilt for not revising enough. IF you're struggling with a specific type of Q on papers, google it. In my day (2009 so basically prehistoric times now) we had bbc bitesize and mymaths. Look at the rationales. IF you still struggle with how they arrived at the answer in the mark scheme, PM me (I have uni exams atm so might not see straightaway - so defo google first). Exam Qs are very very repetitive and GCSE mark schemes actually give you the whole answers (never happens at uni).
I went from a C in some subjects (inc. chemistry) to an A* (well above 90% UMS) with maybe one or two days on the subject. You say you have the knowledge so this is doable. Just be very very concise and strict with yourself on how you revise. Don't waste time overcomplicating issues, GCSEs are much more quantity than quality. Go on the OCR/AQA whatever websites and print out the specification for your course - treat all the points on it like a checklist. For my physics exam I did this the night before the exam (took 3 hours) and got a 99.
Science/maths are short answer Qs so the main things to remember are the basics (esp if you're aiming for a B/C in these). Nail the basics and it makes no difference if you don't even answer the longer more complex Qs at the end of the paper that are designed to target A/A* candidates. Heck, you can even get an A if you answer everything else right. Be confident and self aware - do not make careless errors there's no room for that. Maths is straight forward in that you will know during the actual exam whether your answer is right or wrong (or probably wrong) - make use of this fact.
Finally, look at the specific words in mark schemes. It's possible, and common, to get a C/D in an exam with all the knowledge and revision, but not answering the Q in the required way. This is the most BS part of UK board exams but it's inevitable. Make note of things you come across in the past papers you mark yourself on where you find you said X but the MS says Y. X and Y might be very similar - but learn to use Y.
For
humanities and essay style Qs, it's all technique.
Exam technique:History/English lit - look at the sort of Qs that come up. The amount of marks allocated to each part and the amount of time you have to answer them. I remember for RE and History we had a similar format for the exams where we had to do 4 Qs in 2 hours. The 30 mins for parts a-c we would split something like 12-8-10 depending on how much analysis was needed in each part. Remember techniques like point-quote-explain (PQE or PEE). There are lots of these on Bitezsize for stuff like English Qs - look at them. From what you say, you struggle with structure and answering the Q not the concepts so try to come with a standard idea of how to answer a given type of Qs. Just familiarise yourself with all the content and it will stop looking so scary.
Which brings me to revision technique:
Revision technique:I know people find flashcards helpful (I've used them in the past) but they're good when you have time to make them. IF you don't then don't bother with them. If you need a way to test yourself, I would suggest making checklist of things your need to cover (either from module specification or past papers or your notes) and a separate document explaining the answers. Keep everything short and bullet pointed so it's manageable and not scary looking. Type it up into a word document if it's quicker (you might not have this problem but I used to waste ridiculous amounts of time being a perfectionist with my revision materials and trying to make stuff neat/colour coded/whatever. That's not important - the content is an you need to be strict with yourself on time so you can cover everything rather than 3/10 topics done perfectly).
From the length of this post, you can tell I'm procrastinating from my own revision - I shall head back now. I hope this is helpful. If you have any Qs - feel free to PM me. I will try and give some guidance. I probably won't be able to answer specific Qs on content though - I did GCSEs years ago and probably different spec even then - unless it's maths or a general Q.
GCSE exams start towards mid-late May right? As far as time is concerned: you have enough time, and you also have a lot of things that are more worthy of your attention
Good luck, and I hope you make good use of the time you have so you can be proud of yourself on results day!
It's never over before it's actually over.