What I did when this happened to me was made a list of all the places I felt like I went wrong or could've done better and ranked them on a scale of 1-10 based on how badly I think I'd answered the questions or how many marks I thought I'd lost because of them. Then see how much that may affect your grade based on previous grade boundaries and set a goal for the next exam in that subject.
But overall, there is no use in worrying over a past exam, even if it was the real thing because you're just going to make yourself feel worse without any way to go back and fix it. You should focus on making sure you take care of yourself and do well in the rest of your exams. I'm writing it here like it's easy, but it's not. In truth, I'm trying to do the same thing right now so I understand how it feels, but there really is nothing we can do now but excel in every other exam. I hope you feel less anxious about it and good luck with your A levels.