Much, much better day today. Writing yesterday's post and talking to The Ours about my fears helped a lot.
I decided to change my schedule for a bit - I'd rewrite a minimum of 500 words in the morning, then do an hour of exercise, then write another 500 words after lunch. I managed 1250 words, which I'm really happy with.
I also have the following text on my desktop in huge font:
"What's the worst that can happen?"
"It's better to have tried and failed than to have not tried at all."
Those words kept me on the straight and narrow today - instead of going online as soon as I got up I sat down at my laptop and got writing!
(And thank you all for your comments, they are a great help to me!)