Last year, I got 2 D's in Sociology and English Lit, and a U in Physics. Needless to say, I dropped Physics.
I was devastated, because I love English with all my heart, sobbing on results day, trying to understand how I'd done so badly when I was so passionate and enthusiastic and was doing really well in class, getting Cs and Bs on class assessments.
It's not the end of the world. You talk it through with your teachers at the start of year 13, discuss options - I decided to retake the English exam and, later on, one of the Sociology papers - and work on what went wrong. If you are determined to improve, you can fit in the work needed for the retakes and improvement. I recommend focusing on fixing the weakest area first - which, for me, meant writing an essay a week through till Christmas.
This year I was getting B's in my English and Sociology retake essays, C's in this year's English, and A's in this year's Sociology.
I wouldn't even say that all C's were bad, considering my grades last year...