Rejection is not personal. Writers take it as personal because they consider their work as their identity. Publishing has a 98% rejection rate, often with random, but reasonable, justification.
This is an almagamated piece of advice from John Scalzi who is known for pulling no punches on his opinions, especially about whiny writers. The two most important ideas are "They consider their work as their identity", and "98% rejection date", and that percentage is implied to be "on a good day."
First, I think young/rookied writers often make a mistake of equating their work with them. I consider myself guilty of this, as I put a lot of myself into the story. By that I mean, you'd probably have a hard time distinguishing my main character and myself. These are not Mary Sues though, they're just an aftereffect of the "write what you know" meme. Well, I need a character. Who do I know? I know me! So I'll write me. It's the easiest character to write. That's not to say that the character is exactly me. It's only one part of me, the part that the character needs to be in order to advance the plot. Other characters in the story are also me, but not as much. This is where the problem lies, you identify with the characters because you are the characters.
But think about this, when you read a story, do you ever think, "oh, this character is the author"? No, because you don't know the author on any sort of personal level. That's what I keep in mind when handling rejection. They're reading it the same as I read any other work, based purely, solely on the story presented. If the character is you, then keep in mind they are not rejecting you, they are rejecting the things that happened to the character (and they might be rejecting the character, a common flaw as most people are not very interesting).
That's where 98% rejection comes in. Teresa Neilsen Hayden wrote a killer manifesto called "slushkiller" which provided an excellent 'flow chart' of what could mean the difference between the circular file and the publishing file. 60% of writers get rejected because of something on their side, but the other 40% have to do with gelling with the editor, and what they prefer, rather than an objective view of the competency of the writing, and flow of ideas. I could write tons and tons about rejection, and haven't even experienced any myself yet, so I'll save most of it. But I will say this, when I started trying to work on The Heretic after five years, it was a hell of a lot better in my head than reading it on paper.