As someone who has taken many exams, I know how the questions you didn't know or messed up on play on your mind far harder than those you got right. Assuming that you at least made a fair attempt at the exam, there is a good chance you have done better than you think you have.
Still, even if you have missed on a grade, clearing can vary year-on-year. I have known people who didn't go to their firm because they missed a grade by only a few marks, whereas others have been admitted despite missing a grade. It really depends on a number of factors, cheifly, whether or not the number of people who accept a place after results exceeds or falls short of places available.
My advice would be to accept you can't change your exam performance, no matter how bad it seems in your head, and enjoy a well-earned summer break before you get your results. If the worst happens and you don't get in, then the thing that will pay off is persistence. Be prepared to aggressively ring up universities ASAP to check for clearance - once places are gone they are gone, and the squeaky wheel gets the grease.