It completely depends upon the person and one question: do you think like a mathematician?
Personally, I do not. I got a pretty secure A* at GCSE so I decided to continue with it to AS, which in many ways I do not regret. It was completely new and pretty rewarding once you get the hang of something, but this came at a heavy price. I would do double the homework each week, complete every single paper available and get outside help but yet it was the bane of my studies. I really, really struggled. Around exam season, I finally started banging full marks on every past paper, yet when I went into the exam hall and met questions with difficult twists and new intricacies, I panicked. In the end I managed to achieve 86 UMS average across the AS Level, but I never fully understood what I was doing; plus, when I consider Biology (a subject I only really started trying in just before exam season because I was so preoccupied with Maths before this) and the ease with which I achieved over 90 UMS, it reminds me how much effort Maths really did require. I certainly wouldn't have been able to achieve an A* at A2 unless I pushed myself even harder than I did in Year 12.
But don't let this put you off - there were others in my class that would get full marks on every question after meeting the theory once, and they would only need to brush up on it a few days before the exam. It just made sense to them.
Make the decision based on your own way of thinking - only you fully understand your own mind. But, even if you do choose a subject that you aren't compatible with, anything is achievable if you push yourself and are prepared for lots of blood, sweat and tears :-) x