I agree with the guys saying A Levels are probably a good thing to have. It is incredibly competitive and not having done A levels will possibly make it harder for your CV to stand out amongst the piles and piles of others trying to get the first job.
All airline cadet schemes as far as I know have a minimum requirement of A levels and some also require degrees.
Showing that you can study and apply yourself academically is important.
Additionally, the theory exams are relatively hard - there is a lot to learn and some of it is A level standard.
Not having A levels does not mean you are stupid - of course it doesn't, but maybe you have picked the wrong subjects. I would really recommend continuing with them or, if they aren't the right ones for you, take a break and start again next year with subjects which do suit you and which you can have success in.
Other posters are right - there are many pilots out there without 'fancy' qualifications, but many of them come from military backgrounds, or are older and perhaps had previous careers which set them out from the rest. Nowadays, for your first job I would say at the very least you need decent A level. A degree is more helpful, life and job experience is even more beneficial - basically, you need everything you can get your hands on to show future employers what an asset you will be to their company!