Hi, I had a similar problem on choosing my A-levels. Maths & Physics were not my strongest suits, a careers advisor told me in no uncertain terms that Maths/physics A-levels and an aero engineering degree was out of the question but I knew I wanted to FLY!!!!!!!!
I decided to be headstrong and forced my way onto maths/physics a-levels via AS levels and although struggling and never setting the world on fire as far as results went managed to get onto an Aero degree (via a foundation course) and got a bit more numerate at the same time.
Again, the careers teacher was proven correct, I was almost certainly not cut out for the degree and struggled. I managed to get a degree (just about).
Yes, the careers teacher was correct about me not being well suited for the maths/physics engineering thing, but what they didn't appreciate was that I was determined to fly aeroplanes for a living and that counts for a heck of a lot.
My route worked for me, it cost me a lot of time and heartache but that pales into insignificance when compared to the enjoyment I get out of my flying job. Of the people I work with, many came through the same route (most considerably easier than me!) and would suggest that if maths etc are no problem then go for it. However, and this is a BIG however -Some of the other people I work with have a variety of backgrounds (history, art, geography etc) and they suceeded their own way and are in no way less capable of the work. I can't help but think maybe I could have done something that I enjoyed (History, Politics etc) and was reasonable at for A-levels/degree, enjoyed life a bit more, and still been here -but the ends justify the means.
The important thing is a burning passion to fly, grades are always going to play a part but if you demonstrate your ability and determination to succeed, this will go a long way towards getting a professional aviation career. Get some interesting (aviation related) bits on your CV away from the educational environment and show prospective employers how much you want to fly.
So in conclusion, try to find a route that works for you, it might be maths/physics or it might be theatre studies, at the end of the day it probably won't matter but whatever it is stick with it and don't give up the ambition to fly.
Hope this helps.