Look, youre at the end of Y12, not sitting your final A-level exams. You're self aware that you haven't done well and identified the weakness you have for your subjects. This just means that whatever revision technique you were using is obviously not working for you or you're just using them wrong. If you're bad at writing essays, then you can ask your teachers to teach you the structure and ask them to give you practice essays. When you're done with them, ask them to mark it and give it back. Do it again and again until you're good at it. I did AQA business so if you have any specific questions throw them at me. I also did Edexcel maths and I was in the same position this time last year. I got 2 grades below my target, scoring less than half on that mock paper and I felt terrible. However, this only motivated me more to do better in my next exams. If you're doing past papers without knowing how to answer the questions at first, then you're revising completely wrong. Past paper exam questions should test whether you can answer a question in a different format, not test you whether you can answer the question with little practice. Understanding your maths is just as crucial as daily practice.
Remember that mocks are not the only thing your teachers use. If you know you've done badly in this set of papers, ask them for another opportunity such as giving you another set of papers to do in a few weeks time. This is the time to try to prove to your teachers that you want those grades, instead of not doing anything. If they see that you were truly upset and want to turn it around, then youre more than likely to create a better impression. This just means when it gets to the time to predict your grades, you can have a more flexible discussion with them- if they know you're truly trying to make changes then who knows, they might predict you higher grades.
And, what uni do you want to go to? It's not up to your parents to choose what uni you're going to pay for and spend at least 3 years of your life living there.
My top advice is to not waste time reflecting on things you can no longer change, just think about what you can do in the future to amend whatever youre regretting, such as trying different revision methods- you have the whole summer for this. Yes, its crucial to relax but A-levels require studying outside of school hours, especially since youre doing maths. If youre not some maths genius then realistically spending at least 1-2 hours of maths a day will make a huge difference.