When writing a novel (greatly overrated as a romantic and enjoyable activity, by the way) I always hit the buffers at some point and think the book is utter rubbish. The trick when this happens is to get less serious about what you're doing and recognise that one less novel in the world is not going to make a heap of difference. You're there to have fun and you hope that will communicate itself to the reader. So you take your eye off the ball for a bit, go for a walk, see friends or simply play.
One surefire way to enrage most any artist is to say to them, "Hey, I think you should take what you're doing a little less seriously." This, in the artist's mind, is tantamount to telling a mother, "You should give less of a damn about your kids." POW! Right in the jaw, or in the nethers if you're really unlucky.
Taking yourself less seriously is one of those life skills many people either end up applying selectively or not at all. With artists, the confusion comes from whether to take yourself less seriously or take your work less seriously. The former means being able to laugh at yourself, something any self-respecting sentient being needs to practice with cheery regularity. Life's Rich Pageant is a Veritable Laugh Riot, and attempting to exempt yourself from being the butt of its many jokes only makes even bigger and better gags get lobbed your way.
But to not take one's own work seriously ... what art-hating numbskull thunk up that stunk? Well, that all hinges on what you describe as "less serious". In the end, I know no matter how seriously I take my own work, that only translates into someone else taking it so seriously. This doesn't mean not caring; this means being willing to disjunct yourself from the finished product. You did your best; now it's time to put a notch in your belt and move on to ungrazed pastures (with, one hopes, less cow flop in 'em).
The other day I talked about Vajra with the missus in this vein. Whatever value there is in the book is going to come of what others make of it. I made it, sure; I brought it into the world; and I took it seriously when I did it. But in the end, it's up to the audience what gets made of it. It's that much less mine now.
Put it this way: You have to take yourself seriously enough to know when not to take yourself seriously at all.