The jump is only overwhelming if you think that breezing through the year like in GCSEs will work out just as well. Don't worry, to pass maths AS/A2 all you really need is dedication to the subject - which you seem to have. GCSE Maths doesn't really work as a prediction for your A-level grade, my friend got an A* in GCSE and ended up with a C in AS, I got an A and ended up with 99%, so just work as hard as you can and you'll be fine.