I've just come back from my German written AS, and have had my French oral, German oral, and Photography exam already, 2 more to go. Even when you know that you know enough for the exam, you are always stressed. If this is your first, then I don't think you should be worried, I mean, it's important, so it's stressful
Mocks often tend to come out worse, because they're taken so early in the course. This means that people haven't even covered half the stuff that will likely come up in the real exam. Take this, for example, we were studying All My Sons by Arthur Miller, but had only just started it after the first book, and we were still examined on it. That isn't fair, and it's only likely that the results don't justly reflect the accumlated knowledge he'll have at this point.