This is not my own, but the person I learned it from can't remember where she first found it. And it's not a direct quotation, because I've modified it to sound better in my own ears. But the sentiment is exactly the same.
"A writer is a writer not because he has amazing talent. A writer is a writer because, even when nothing he does shows any sign of promise, he keeps on writing anyway."
google search shows the following:
A writer is a writer not because she writes well and easily, because she has amazing talent, or because everything she does is golden. A writer is a writer because, even when there is no hope, even when nothing you do shows any sign of promise, you keep writing anyway.
– Junot Diaz
Thanks. I see that even before it reached me it had been substantially modified. Writers are also editors.