Yea 12 mocks stress

Do your best in the time available.
Apply excellent exam technique. Not too long on 1 question. Answer the question, not what you'd like the question to be asking. For essay type questions write lots of little bits about different things, instead of a lot on one or two things (it's how marking schemes work).
Aim to get a good night's sleep before exams. Get into the habit of going to bed at a certain time and getting up at a certain time a week before exams.
Last minute and night before revision is the most helpful, due to how our memory works. Short term vs long term memory.
Fidget in your exam chair. Keep that blood flowing to your brain and writing hand.
Treat it as if you're playing a complex board-game. You're playing to win, but if you lose it's not the end of the world. This should help give you the mentality of positivity and enthusiasm whilst damping down the nerves.
A far far better goal to have than getting A*AA is to be earning more money by the time you're 25 than your parents and secondary school teachers. And to be earning it by doing something that you don't hate.
There's plenty of ways you can do that without a single A level and without a degree.
Let your parents say whatever they want to say. Live for your own approval, not theirs.
At the end of this academic term, turn your attention to your Plan B. That of how you can have a great life without academic qualifications. And start embarking on your Plan B. Then, in September you can go back to your academic Plan A if you want. Or have Plan A and B going in parallel at the same time.
Don't be like a typical teenager that only has a Plan A. Because they're ill advised and lack the experience to realise that Plan A is a pretty sucky plan.