I'm not a good writer. I used to be a good writer, when I was maybe five or six. It was probably a low point in my writing ability technically speaking; my letters backwards, spelling poor, and vocabulary very not good. But I loved it, and I wrote a lot, and shared it with the people around me. Then slowly, for some reason or another, I stopped writing and somewhere along decided I wasn't any good at it and so I should just stay away from it.
I used to think I wasn't any good at a lot of things. I used to think I wasn't good at sports, and running in particular. I'd eventually find out I was asthmatic and all of those years I'd thought everyone else was naturally better than me I was suffering from a treatable illness. I was never any good at math either. Towards the end of school I started getting private math tutoring and pulled my grade from a D to a B.
Worse than assuming you can't do something is someone else assuming it for you. I was told I wouldn't be able to study computer science in college because I only got a B in math. I should go on to science, the thing I had always been told I was good at.
There's this insidious idea we teach our children: natural talent. It's awfully good at convincing you you'll never be good at something when you see others praised for something. It's also pretty good at convincing you you'll never been good at something when you're praised for something else.
As it turns out I can do math and programming, and I can run races. I wasn't any good at first but with a lot of work I'm getting better. I'm not a good writer. I know this because I know the amount of effort that goes into becoming good at something. I'm not a good writer but now I know that nobody starts out good and all it takes to become good is practice.