Your memory is a monster; you forget - it doesn't. It simply files things away. It keeps things for you, or hides things from you - and summons them to your recall with a will of its own. You think you have a memory; but it has you!
I write the last line, and then I write the line before that. I find myself writing backwards for a while, until I have a solid sense of how that ending sounds and feels. You have to know what your voice sounds like at the end of the story, because it tells you how to sound when you begin.