I have about 20 half-finish novels sitting on my computer, in my filing cabinet, or under my bed. The story with each is the same - I start out with a full head of steam and great ideas, but about midway through I get bored. I've told the story about a million times in my head, but it moves faster up there than I can write or type (admittedly, I can type pretty fast, too). By the time a story's been through its thousandth re-telling, I get tired of listening to the characters and jotting down notes about their lives.
I get bored with the story, shelve it, and move on to something else.
The problem with this trend is that it's disillusioned me to writing anything new. I'll carve time out of my day for writing, but I just can seem to get started. A voice in my head mocks me, reminding me "it will just end up like last time, forgotten in some envelope under the bed." So I give up, let a story mold and rot in my head, and go for a run instead.
So, what is the best way to keep myself interested in my own story while my fingers struggle to catch up to my brain? What's the best way to prevent boredom from killing a novel-in progress? How can I preempt this negative creative trend before it evolves into perpetual writer's block?