This depends - how good is your teacher at the moment? At GCSE, I had a very nice teacher, but we had maybe three lessons in the two years because he always completely ignored us. When it came to exams, I had to learn everything by myself but ended up getting two As (it was a twin GCSE), one of which I retook after getting a B.
I am now doing A-level Maths after joining the course a month late and having learned nothing at GCSE, and my teacher is pushing me to do Further Maths next year (which I will definitely do), and possibly a Maths degree. I usually get about 68/72 on papers, and I'm confident that I'll get an A this year.
Basically, if you think you're not doing very well because you're not being taught very well, DO consider A-level Maths, because it might all turn around.
However, if you have a decent teacher but are still struggling, you can always do your absolute best now, then take Maths as a 5th A-level next year and drop it if you're struggling.
All in all, I'm personally finding A-level maths easier than GCSE, but, again, that's probably because of the teacher thing.
If you're still unsure, maybe speak to your teachers to see if it's worth trying.