The failing writer is more honest and self-reflective than the successful one because once he tastes success the tendency is to assume that every word he bangs out is wrought in gold. The failing writer goes back on every word a hundred times and even the 101st time he is convinced that his brain is made of liquid shit and the carcasses of diseased rats - how else can such pure drivel erupt from one's fingers?
That under-confidence and crippling self-doubt, if left alone (and alive) for long enough can create great things. Unfortunately that under-confidence and crippling self-doubt also affects other parts of his life and more often than not the failing writer either gives up or puts a shotgun in his mouth before he can finish what would no doubt have been a great piece of writing.
A confident writer once he has felt the heady buzz of success can respond in two ways - either he remains on Earth and uses his success as evidence that he can, if he wants, create good work if he tries hard (which is rare). Or he can let the success get to his head and write everything as if, the moment he received that e-mail from a publisher stating the publishability of his work, he was possessed by the spirit of Vladimir Nabokov. And if this is the case, everything that flows out of his brain, through his fingers and onto the screen is platinum class, whereas, in reality, he is putting out half-assed garbage, without the stringency that self-doubt and low self-esteem guarantee.
Another reason the failing writer is more honest is that after many, many years of writing and nobody giving a shit, he stops to give a shit about others giving a shit, not out of some hardening of character, but rather out of sheer exhaustion. Once he stops giving a shit about what people think, he taps into a deeper (and often darker) part of his psyche. He begins to form a twisted mass made up of his reality, his fiction, his mind and his body. Soon he is no longer writing a book - he is just trying to climb out of a grave that he himself dug, in which he buried himself.
Thus writing becomes a way of saving himself, of getting through the demons of the day.
The successful writer is already out of the grave, has already saved himself from himself, and even though it is a cycle - because eventually even the successful writer will find himself in some dark hole and will have to save himself again - one can expect a few books that pander to 'target audiences', full of lazy cliches and weak words.
So if you are a failing writer, almost paralyzed by self-doubt and no esteem, keep at it - once you're done with that work give it up to someone who you know will send it to a publisher (because you yourself can do no such thing) and you might be surprised how successful it turns out.