In a word: Massive!
In a few more words: Massive but not insurmountable if you're really keen.
I used to teach maths (to both GCSE and A-level) and I always warned my able Year 11 students that, however easy they might have found the subject at GCSE level, they should definitely expect to struggle at times at A-level. I also repeated that warning at the start of the A-level course. As I predicted, even the most able students (who went on to achieve grade A at A-level and even first-class honours studying maths at university) suddenly got a shock when they first encountered some of the A-level material.
I'm sure though that, if you're keen enough, you could reach a good A-level standard, Sandy, but don't expect it to be easy!
Remember that there are several maths teachers (either past or present) among the regular contributors here on AB, so there should always be some help available when you need it.