Faking it

Why do I always feel when I'm writing as if I'm faking it, trying to convince people that I know something that I don't, that I can enunciate a truth that I really don't know to be true, invent it with words that make it sound convincing. I feel a tremendous lack of confidence in what I write. It has a fascination for me and yet I fear it; it's like the firing squad, a moment on the limits. Of....????