Yup. Breathe. Seriously - breathe in, out, get some tea and cookies, breathe some more.
We all do it. It's a prereq for being a writer. Eventually, you WILL have a bout of the Am I Crazies. So after you breathe, remember: it's normal. You are not alone. We've all been there.
Now then. Calmer? Good.
Answer the following questions as completely and honestly as you can:
-What do you love about a good book? What makes you crave them? Why do you slog through books you don't like to find a jewel that you absolutely love?
-Why did you start writing in the first place? What do you have inside of you that you must share?
-What is it about your stories and characters that makes you so passionate about them?
-Why have you kept coming back to the keyboard day after day after week after month to write this story?
Take as long as you need to find the deepest, truest, most honest answers to these questions. Even if it takes days. Don't settle for the trite "I write because I love to tell stories." Go deeper - why do you love to tell stories? Why did you choose this one? Why have you kept going all this time?
Have you realized how much writing is a part of you yet? Is your little inner Muse curling around in joy thinking about books and writing and the thrill of a perfect phrase, the excitement of seeing your vision transfer from your mind to paper to another's mind? Good.
That's what it means to be a writer. Like Matt said, a little craziness does us writers good. So are you crazy? Probably, just a little. But so am I, and so are the rest of us. Embrace the craziness. What we need to get rid of are the doubts, speculations, self-deprecations, and the slings and arrows of outrageous fortune that try to shoot us down.
Don't think about publishing. At all. No agents or editors or publishers or readers. They don't exist right now. It's just you and your writing. How do you feel when you're having one of those awesome everything-I-write-is-just-flowing-perfectly days? THAT'S what we need to remember and hang onto during the Am I Crazies. Don't fret about whether you book will ever make shelves. That's not important. Writing is what's important.
Still not convinced? Still suffering from chronic Am I Crazies? Then step away. Take a sabbatical. Quit, if you must. But when the spark comes back, don't push it away. Come back. Write. Edit. Love being a crazy writer. (And have more cookies.)
Inspiration isn't about the muse. Inspiration is working until something clicks. ~Brandon Sanderson