Rejection isn’t good – but it’s helpful. I remember a quote, from Michael Frayn’s Clockwise – ‘I can take the despair, it’s the hope I can’t stand’. It’s a bit like that. A definite ‘no’ means you can stop worrying that it might turn out to be a ‘yes’. It’s helpful, in that in means you know that it’s time to move on (though by this point, you’ll have moved on to a dozen different projects anyway – it’s more a case of moving on psychologically. Letting it go.)
Writers have an odd psychology. I’m not claiming it’s unique, it’s probably common to all creatives and elsewhere, but it’s an odd combination of being massively self-critical and massively self-confident. Which sounds like a contradiction but isn’t, because the trick is to alternate them, not do them simultaneously.
In my experience, the best writers tend to be massively confident in their own work – you have to be to get it out there, take the knocks – but the reason why they're massively confident is because they’ve been up to five in the morning the night before agonizing over every syllable. The confidence is born out of massive self-criticism; similarly, the reason why they stay up to five in the morning wondering whether dashes should be semicolons is because they know the end result has to be worthy of the great confidence they’re going to place in it. It’s reciprocal, sympathetic. The greatest writers have both qualties, not only in abudance, but in balance.
Bad writers tend to be either self-critical without self-confidence, and never get anything finished or sent out, or self-confident without being self-critical, and never get anywhere because they assume their first drafts are works of genius when in fact they are a dozen or so re-writes away from being barely readable.