Don't do maths A Level just because you got an A/A* at GCSE, it's really not the same, and there are plenty of people that got A* at GCSE and can't hack it at A Level. You've got to really enjoy maths AND enjoy the challenge that it brings. And you've got to be prepared to work through your confusion an keep on going for the long haul.
But, if you enjoy the challenge, really enjoy doing maths, but can't stand the idea of doing Further Maths as well, rethink at least, research. That's my big mistake, and the thing is, everyone told me to think about it but I didn't.
But then again, don't dwell too much on your mistakes.