Having gone from a rubbish maths phobic to getting an A Level and some Uni Maths courses under my belt I can tell you for certain that you're not crap at maths!
Its acceptable to say you can't do maths in this country and so many people have convinced themselves that it is hard and they can't do it that it becomes a self fulfilling prophecy.
If you're of normal intelligence, have no conditions such as dyscalcula, can count, understand addition, subtraction, multiplication, division and can follow and remember instructions then you're more than capable of getting a good GCSE grade and beyond.
Maths is nothing to fear its just numbers and you have to get yourself past that. You can learn it and it's a total revelation when you do.
You must go back to the very beginning though and fill in little gaps in your knowledge. Many people are lacking some of the basic rules, BODMAS is a big one - there is even a whole thread in classics showing how many lack this simple and basic rule, it's these gaps that can make maths incomprehensible. I bought the learn at home books (CPG or is it CGP - am on phone so can't check are really good) but you must start from key stage 1 and work through from there. You'll race through in no time filling in the blanks.
A great way to improve your maths dramatically is to work on your mental maths. That means learning those times tables by rote until you know them inside out. It increases your confidence with numbers and makes other operations quicker as you already know that 6 x 12 = 72 for example and don't need to waste time thinking. I also started to add up my shopping cost in my head. Start off with rounding to the nearest pound then the nearest 10 until you can do it to the penny. Wherever you can abandon a calculator and do it in your head or using a pen and paper. It is really effective so don't skip this step.
It has lots of benefits. I know exactly how much my shopping will cost when I get to the checkout. I can instantly work out how much of something I will need and which offer is the best deal. I also won £100 and convinced people that was a genius mathematician with my ability to find the square root of 4 digit perfect squares instantly, its nothing special just following 2 simple rules using maths taught at primary school.
I did it and you can too. Its so liberating to be good at maths and it does put you head and shoulders above others while increasing your self confidence dramatically.
Good luck with the course.