Pithy advice from babthrower !

Unfortunately you don't say what subjects you are doing . For current purposes, I've assumed they are arts subjects.If they are sciences, only some of what follows is appropriate (you'll have to work out which bit

)
The first, good thing, is that the mocks have given you a bit of a shock.Now you have to find out, with the aid of your teachers, why it is that you fell below your expectations. At a guess, your exam technique is faulty, for one thing.That's the easiest thing to cure, too.There's a definite technique to writing an exam answer or essay.(There are whole books on this subject)
For example: Remember that the examiner is looking for a number of points to be answered in the essay. They all count for marks, usually equally. Missing one costs you a lot more than anything you gain by writing at length and brilantly on any of the others.Make sure you've jotted down all the points raised and required as a note for yourself before you start, double check, and then write the essay,ticking each point in your list as you go but be very sure to cover them all, however hurriedly at the end (shouldn't happen: you should pace yourself, with an eye on the time

).
It helps to put yourself in the shoes of the poor examiner, reading dozens, or hundreds, of scripts. Imagine what's it like to have the same answer citing the same text, quotes and opinions over and over again [the same because they've come from the same source, be it a textbook used everywhere, or the internet] He or she likes an interesting opening [and above all does not want a recital of the question as an opening]. In a history exam, it's quite agreeable to have a pertinent quote from a historian, or, better, a contemporary or participant in the events, as your opening line and develop from there. If you want to see how it's done, have a look at a serious newspaper like The Times.Their reporters and their feature writers all catch the eye and mind in an opening line or two, which says briefly what the ensuing story or piece is about, and then develop logically from there, but the opening lines are always critical in engaging and keeping your interest. And if you can get your hands on an essay or book [eg. The King's Peace] by C V Wedgwood you'll see it done by the greatest historian of her day. Read her opening lines and you are hooked for 600 pages! The same principles apply in other arts subjects (and in economics, come to that !)
More work is definitely needed on the subjects, but your teachers are the ones to help there.
And the answer to your question? What's done is done. Forget about it.You can't change what has happened, good, bad or indiffferent.
However I doubt very much that your mock results get anywhere near your intended sixth form college. Whether they cause your teachers to revise their view of your likely results, who knows, but that too seems very unlikely. The whole purpose of mocks is to give
you a scare, not the teachers

. They cynically know your potential, and the mocks won't change that knowledge.
You may guess correctly that I'm a father who has been through this, at all levels of education. In my case, most recently with my own daughter and her classmates (and their parents. Believe me, we are all in it together

) And I've done a good few exams in my own time (some of us old people have and, amazingly, nothing has changed much).