Hey, don't worry. I did really well in my end of year 10 mocks and then really badly in my year 11 mocks (at least for me- I go to a grammar school so there's a lot of competition and pressure). I got a d and a c- I'd never got one before and it really showed how important revision is. For my year 10 mocks I revised loads and did so well and then in year 11 I basically did none. However, this really motivated me. Remember that year 11 mocks will often be a lot of the course and you won't have learnt it. My school treated them seriously but if you do badly it really motivates you to revise and get help on any areas you're struggling in. Btw I got 9A*s and 2As in my GCSEs. You'll do great, if you're posting on here you obviously care. Don't let yourself get too worked up though, I had panic attacks and cried several times over mocks and after you've got your GCSE grades you will be able to see that they meant nothing and really aren't important other than a means to help you revise.