Whoever said that short stories are easier than novels has no bloomin' idea, I say.
As far as I'm concerned, the real work of writing is done not at the desk, but beforehand, inside your head (well, mine, anyway): you need to start dreaming a piece of fiction before you can put it into words on the page, you need to see it, smell it, hear the characters talking, be fascinated by their bodily movements, get obsessed so that you can't help thinking about the damn thing, it just won't leave you alone. Which, when you're writing a novel, is great: it's what keeps you at it, those long months it takes to write it. It's also great, of course, while you're writing a single short story.
But if you're writing a collection... Well, you need to keep dreaming up new worlds over and over, and in order to dream up the next one you need to let go of the last. Which I'm finding really difficult to do at the moment: I've got a new story to write, and I know it has the potential to engross me utterly, but the last one is still trailing me, my mind keeps slipping back towards it, the old dream still following me around in a cloud.