Don't doubt your abilities! How good you were in year 9 doesn't matter, what matters is how good you are right now and how much you've improved. Don't worry, lots of Cambridge students are perfectly normal. Pushing yourself is a good thing, it's exactly what Cambridge wants to see. Not to mention, the kind of maths done in year 9 is DRASTICALLY different from the maths done in a CS degree. Please apply, and all the best!