The only thing that is similar to the real GCSE exam is the environment that you are in. I feel personally that in your mock the pressure I felt to do well was quite high but the anxiety before my really exams were a lot worse. You have also got to take into account that you haven't finished many of the course by December so realistically do you feel this is a true representation of how you are going to achieve? also if you still do course work ,not to sure with all the government changes , this will definitely play a role it could either lift a grade or drop it and this is not taken into account during mocks. I want to be honest and say that I worked hard for both my mock and GCSE but my mocks were better I only had to do one paper or two for each subject= 11 or so exams but in real exam I had 20 papers to do (stress).
Mock
Bio -A*.
chemistry-A*.
Physics -A*.
geograohy-A*.
Graphics -A.
Maths-A.
English lit-A.
English lag-B.
I got (in same order) A*A A A* B A A* A
I had the same worry about the exam being hardest the year I was doing it but, if it is everyone will think so, and grade boundaries will change accordingly. Your year is no different to the previous year so your paper will be very similar and you will recognise styles of questions I promis you.