Another really good piece of advice from Bird by Bird by Anne Lamott, a great little writing book I'm reading now:
The moment you feel overwhelmed by writing a scene or a chapter or - good lord - an entire book, tell yourself that all you have to write at that moment is enough words to view through a one-inch picture frame. This is probably equivalent to the first moment we meet a character, opening a door and walking through it, or a few lines of dialogue between a man and a woman thinking about breaking up, or a lonely dog looking in the windows on a cold night. You get the idea. It's a snippet. A step. That's all you have to do, just enough to fill a one-inch frame.
Lamott writes, "You don't have to see where you're going, you don't have to see your destination or everything you will pass along the way. You just have to see two or three feet ahead of you. This is right up there with the best advice about writing, or life, I have ever heard."
So soldier on, and keep this in mind when you're feeling overwhelmed at that keyboard. One inch at a time. That's all. Happy writing!