Hi there! I'm a year 10 student with Gcse mock exams coming up in a week from tomorrow. I am in great stress and I haven't been organising my time well enough. Like you, I haven't done much revision. I have studied so hard all year long to try to achieve good grades for my end of unit assessments, but in reality, I always end up not getting the grades I wish for. I have so much to revise for in little time (less than a week) and I have barely started. I feel so burnt out and stressed, but I can't even imagine how you feel as you are taking A-levels and my current qualification is only at pre-Gcse stage.
What I would recommend (from my experience and others) is please, please, please don't pull any all nighters!! They burn you out completely and although you may feel like you have so much to do in little time, and that pulling all nighters will help you, in reality they wont. I have done this in the past due to immense amounts of stress and they really didn't help. When the days of the exams come, you will feel super tired and drained from all of that time spent revising, when you could instead sleep and help your brain recharge and rest. Let's face it, our bodies need sleep every night for a good amount of time. If you stay up trying to learn or revise something, you probably won't remember it the next day anyway or it may just slip out of your head, so sleeping is the better option. This has actually worked for me instead of staying up all night even though I have barely revised anything.
I really do understand that you have exams tomorrow and I feel so sorry for you! What I can say is please do not worry that much about your exams. Try to get a good nights rest and calm yourself down! Although you say that you may not be entirely prepared (which happens to countless people, including me), think about the revision you have done as opposed to the revision you haven't done, and with the amount of revision you have done, you'll be more than ready! Try to remember what you did, even if it was just a little part of a topic, and if you feel really stressed about your exams tomorrow, just tell yourself that you have tried your absolute best! I know that feeling when you know you haven't revised that much and you come up to your exam and feel so anxious, but in your case, these are only mocks! try to get a feel of what the proper exams will be like and try to at least be thankful that these aren't the real exams. You'll have a chance to do your real exams to the best of your ability.
On a final note, what I would say is please don't stress out! You just have to try to make yourself understand that you have been trying your absolute best during the whole year, so this doesn't mean you are lazy or you didn't put enough effort in. You have put in so much effort and you should feel so proud of yourself! Try to tell yourself that you have been feeling drained out due to the fact that you *wanted to* put in maximum effort towards your grades, and that these mock exams don't define you! They are just a guide and another step into the path of your education and uni applications.
Sorry for the long response, butr I felt that you should deserve to hear this! And don't forget, YOU GOT THIS! I believe in you!! : )
Stay positive and strong!