You've done it now, so there's no point stressing over it. For you're really worried about it go and speak to your careers advisor or someone at school about what options would be available should you not get that C, so you can be prepared if you don't achieve it. What course is it you want to do next year? Because there is a chance that if you speak to the sixth form/ college or wherever it is you would be doing it they may let you still do the course with less than a C, or take the course whilst retaking your English GCSE at the same time.
But if you've looked into it then just wait until you get your results and focus on your other subjects. When you get you results then you'll be able to explore your options more and you might even be pleasantly surprised at what you got, because you could have thought that the exam went worse than it did because you were panicking!