Was absolutely convinced that I'd failed english and maths - ended up with a B in both (never got above a low C in maths before).
Failed 2 subjects but got Ds and I was fully expecting Fs.
Biggest surprise - I'd never passed history before and was on a D for years, and came out with full marks in one exam and got an A overall.
Take this from someone who's been there - you'll do so much better than you think, don't underestimate yourself! Also, GCSEs aren't as important as you think right now, you'll realise that in a year, so please don't stress yourself out with grade boundaries or anything now. Good luck!