Writing is a process. It's not typing.
Your first draft is a beginning. It's not the end. Every book you see on bookstore shelves has gone through many drafts.
The FUN of writing BEGINS after your first draft is complete.
Once you've written the draft, it will always be far removed from your original conception of the book. That's fine. You've got a chance to make the book better than your inspiration with your next draft, and the one after that.
Write about your book
When I'm coaching writers, their biggest challenge is knowing how to revise. There are many books which will show you how to revise your novel.
However, if you'll trust yourself, you'll be your own best guide through the revision process.
Talk to yourself on paper -- write about your book.
You've written a book, or you're in the process of writing it. You're disappointed. Somehow, the shining vision of your book that's in your head doesn't translate to the page.
You SHOULD feel as you do. Feeling that your writing is bad is a huge accomplishment. Otherwise there's no hope that you'll improve the book.
A draft is just a draft. In the post above, you'll find a way to make your "bad" book much better.