Determination and hard work are both qualities that would make A level study alot easier. The step up from GCSE is insane, and so is the increase in subject content, but as you settle in you'll get used to the work load. Studying at A level is definitely not as impossible as you managed to make it sound.
Having much more independence and the fact you got to choose the subject areas you're studying (and so you probably have an interest in them) make it easier too.
It depends on what subjects you are doing. Also like the above person said, you can pick your subjects this time, and there will be a couple or no compulsory subjects depending on your college, unlike GCSEs where most of your subjects are compulsory. You can also drop subject(s) after AS level, but at the end of A2, you should have done 3 A-levels.
Its a big step from GCSEs, you may need to revise better, do more modules, more homework etc.