Long before I became at author, I was a maths teacher and, since I left the classroom, I've helped quite a few people with maths on a one-to-one basis. Over the years, I've heard the phrase "I can't do maths" many times, but the people who said it were always wrong. Their problem wasn't that they couldn't do maths – it was that they hadn't been taught it well enough.
That wasn't necessarily the fault of their teachers. I know from experience how hard it is to give individual attention to each student in a large group. You have to decide to move on to the next topic when most of the class understand what they are doing – you can't hold them all back because one person has failed to grasp a new topic or has fallen behind because they've been ill.
That doesn't matter in most subjects. You can still learn about the Second World War, even if you've failed to learn about the first one, and not having read Private Peaceful doesn't stop you studying Of Mice and Men. Even in science, you can learn about plant reproduction without having mastered human nutrition.
Maths doesn't work like that. The individual topics build on each other like bricks in a tower so, if one of the lower bricks is missing, the higher ones wobble or fall down completely. Then another difference between maths and other subjects kicks in: it's possible to get it completely and utterly wrong. And there is nothing as effective as a page full of crosses to make someone declare "I can't do maths".
This problem is heightened by the tendency for schools to save the most qualified maths teachers for the older and/or more able students. In primary school, maths is nearly always taught by teachers who aren't mathematicians. However well they teach, they don't always lay the best possible foundation for future learning because they don't understand which bricks are are the most important.
For example, children spend many hours in primary school mastering addition, subtraction and multiplication but often spend far less time on division, although mastering it is equally important. And when they learn about fractions, they spend ages colouring diagrams showing that ¾ means three parts of four without anyone telling them that ¾ also means 3 divided by 4. That second definition is vital to tackling more advanced topics such as algebra, and students who haven't learned it will struggle with maths through no fault of their own.
So if you think I can't do maths, start saying I haven't learned maths instead. If you go back and fill in the missing bricks in your tower, you'll find maths is nowhere near as difficult as you thought. And when you start getting it completely and utterly right, you'll find it can actually be fun as well.