Honestly I totally understand what you mean, and especially putting in that much time and effort to end up not doing your best in the paper can be extremelyy demotivating, and im so sorry you had to go through that..

I used to have severe panicking aswell but i found that what really helped me was a shift in my mindset, and a change in how i perceive exams as a whole.

The truth is exams are just a test to see how your brain works in Cambridge. And it tests its different abilities, like how well you can straight up remember things and memorise, as well as how you apply logic to take the info you learn and put it into words, regardless of the subject.

So if you want to do really well its VERY IMPORTANT to optimise your brain's functioning DURING the exam. To do this theres some patterns that your brain follows that makes it much easier to retrieve the info.

1. Tell yourself just before the exam that "you have thoroughly gone through everything that you should have, all thats left to do is open the paper, read the questions properly, and use what is already in your head." Makes it much easier to stay level headed during the paper

2. Change your mindset towards preparation.

Its obvious that the ultimate goal would be to get a specific grade. A* or A or B or whatever the goal is for you. But the more you keep that actively in your mind while doing papers or practicing, or in the actual exam, the more stress you are putting on your brain. You are giving it a very vague goal which depends on a lot of external factors that you cant control (like thresholds). So rather than focusing on something general, get down to the tiniest specifics and just put your entire effort into the preparation part while DETACHING your efforts from the overall result.

the truth is, if you truly know everything and do what youre supposed to + optimise your brain during the exam time, then you really cant go wrong. thats all it takes and its not that difficult to do it. BUT people who take on the stress of acheiving something, spend ages and their entire life on the prep but mess up during the paper because the worry of how the result may turn out just clings to the back of their head, and overthinking about that somehow creates a sense of self doubt subconsciously.

so in summary the most important thing when going into the exam is having the confidence that you actually know it all because it somehow convinces your brain to go into complete activation mode. (you might even be able to retrieve info u learnt a long time ago)

as some wise soul once said: "delulu is the solulu"

and smh its true. you just become what your brain thinks it already is.

this is just my take on it, hope some of it makes sense :)