I use a pottery analogy for writing: You can't throw a pot until you put some clay on the wheel. Writing is just putting clay on the wheel. Write and write and write and write and - even if you think you might be off track, or if it isn't working: WRITE. Once you get a corpus of words all arranged on the page, you can finally get down to the story. Figuring out what the story is (because it is rarely what you thought you were writing!) comes from editing and rewriting, and moving scenes around, and making that character female instead of male, or moving the whole thing from a space-themed thing to an underwater thing, or whatever. Until you have some clay on the potters' wheel, you can't make a pot. And yeah, it sounds trite and unproductive, but you don't get more words on the page to work with, until you write them. So just write. (And maybe go for a walk. Or have a glass of whisky.)