At that point, I have no choice but to write the story. The itch becomes a burning rash inside my mind, and as I write it becomes more intense. Sometimes work and social demands keep me from writing, but the itch to complete the story is always there, demanding the lidocaine that can only be provided by writing the last sentence. If I make writing sound less than fun--good, because it often is. I tend to advance in frustrating fits and starts, a fit of inspiration spawning a flurry of writing followed by a painful pause where my mind tells me that what I have written is crap, that it is going nowhere, and the entire premise is flawed. Then inspiration strikes again, and the story advances.
I wrote the final sentence of a story last night. The itch has been scratched; my mind is at ease. For now. The story is by no means finished. This is just the first draft, and there is much more to do. It'll go on the back burner for a few days, then go through a couple of rounds of rereading and editing. And the itching will begin again. For some stories, it's a relatively painless itch--a few wording changes satisfy it quickly. For others, it requires a painstaking revising process, including moving, rewriting, and deleting whole scenes. I have the feeling this one will be closer to the latter scenario. From there, it'll go out to my writers group to be scrutinized, torn apart, and again rewritten to fix flaws that I missed. Then, finally, my mind will be at ease. By then, of course, I'll probably be deep in the grip of another story. I already have an idea coalescing.
But at least for today, I have written, and I am at ease. For now.