A lot in writing is about having faith -- in yourself, that you'll be able to finish your novel; in your writing, that it'll get better with practice; and in your ideas, that they'll come together in the end.
I am editing my third novel now and while I'm excited about it, I often ask myself if it's good enough. The answer: Who knows? Writing is such a subjective thing that while one person may love it, another may hate it. The only thing to do is to push through any self-doubt towards the feeling of accomplishment when you finally finish and you're happy with the end product -- regardless of what others may think. Easier said than done!
I've found both chocolate and wine to be of assistance in overcoming self-doubt. I'd be interested to hear what gets other writers through!