Do be kind to yourself. Fill pages as quickly as possible; double space, or write on every second line. Regard every new page as a small triumph. Until you get to page 50. Then calm down, and start worrying about the quality. Do feel anxiety - it's the job.
When I'm writing I just think there's only the page and me and nobody else.
Schools don't really allow failure and yet it's part of any endeavour, not just writing.
I write short stories when a little idea occurs to me, that I know isn't a part of a novel that will stand by itself and should be concentrated.
I tend to plan as I write. And I want to leave myself open and the character open to keep on going until it seems to be the time to stop.
When I was growing up, the exam system didn't allow you to write fiction, so you never did.