Julie Wright say,
"Don't beat yourself up. The publishing industry will do it for you."Which, of course, reminded me of Bob Dylan's, "Everybody Must Get Stoned," and my own comments about the great chain of rejection that is the publishing industry.
But I think there's more to this than a lets-feel-good-about-ourselves moment.
To begin with, there's clearly a difference between self-criticism and beating oneself up. Where the latter is about grief spirals and pity parties, the former involves a realistic assessment of where you are and constructive plans to improve.
There will, naturally, be people who hate your work and loath you, often for reasons entirely beyond your control. But the number of unambiguous foes, who would gladly beat you up if given the chance, is dwarfed by the vast majority of people who inadvertently beat you up because they need an excuse to not pay attention to you.
You can understand this behavior in terms of the query problem. An agent who gets thousands of queries a year when they might realistically be able to take on one or two new clients doesn't open each query hoping it will be the one. They're looking for the quickest way to determine if it's something they can safely ignore. It's nothing personal. It's simply the most rational way to deal with an avalanche of material.
So beyond the simple psychology of a positive attitude, if you don't believe in yourself, who will? If you're not your own best advocate, who's going to do it for you?