We've all felt like that at some time or another. A writer called Gene Fowler once said, "Sometimes I think it sounds like I walked out of the room and left the typewriter running." Feel familiar?
Usually I find I'm convinced it's complete rubbish when I've just finished a particularly tough scene which felt like it had to be at least 3000 words and I discover there's a mere 1000 or less. That's because it was such a slog getting there. And now I'm sure the mere 1000 is going to end up mashed in the recycle bin.
Worse is the time when it's flowing wonderfully well and you feel on top of the world as you realise you've exceeded your target for the day and you're convinced it's the best thing you've written this year. Then you read it back and deflate like a pricked balloon. It's terrible. It's the worst writing on the planet, and you immediately realise you've missed your vocation and nobody is going to want to read anything you write ever again.
Oh, those writing blues! How does one get over these humps? First off, you're allowed to write junk. Who said everything you set down has to be perfect prose? Where's the blueprint for the writer who does it right every single time? There isn't one. So Rule One is - give yourself a break, don't beat yourself up.
Rule Two? Leave it. Don't try to edit it now. Your judgement is out the window. You've gone into effect of your writing instead of being cause over it. You'll be wearing an editor's hat that's the equivalent of Simon Cowell laced with Craig Revel Horwood. Forget it.
Come back to it next session. You'll surprise yourself by discovering it's not nearly as bad as you thought, and a few tweaks will put it right. In the unlikely event it is still junk, cut it from the document and save it in a "temp" file, and then start again from the last good point.
Warning: Don't delete the junk! You might need it later. Maybe not the whole thing, but snippets. And if you don't need it, by the time you realise it, you'll have plenty of new quality material and you won't care about chucking it out altogether.
And take a leaf out of Katherine Mansfield's book: "Looking back, I imagine I was always writing. Twaddle it was too. But better far write twaddle or anything, anything, than nothing at all."