Rejections are part of the deal. Everyone who discusses pro or semi-pro writing says this (and if they don’t, they’re selling something). If you submit, you’re going to get rejected, and it’s probably going to happen repeatedly. It happens 
for any number of reasons: the piece isn’t a good fit, it’s just not to the editor’s taste, it’s good but it’s competing with other pieces that are better, or–and this sucks but for a lot of the time and for a lot of people it’s unfortunately true–what you submitted just wasn’t as spectacular as you thought it was when you banged it out.
If you’re going to keep submitting, rejections are something that has to be prepared for and dealt with, as much as either of these things is possible. Everyone’s coping mechanisms are different, but here are the things that I find work best for me when I’m facing the classic “Thank you for your submission. Unfortunately, we have decided not to accept…” response.
1. Let yourself be upset. No one likes getting these things. What it amounts to is “sorry, you weren’t good enough for us” and that is never a nice thing to hear, no matter how gently phrased. And rejection slips tend to not be gently phrased. They’re usually not harsh, but with the sheer number of them that editors have to send out, they don’t have the time or the energy to be responsible for tender author feelings. It stings. That’s okay. Allow some time to let it sting. That said, don’t wallow in it. Allow some time, but be strict in the amount of time allowed. Once it’s up, let it go.
2. Don’t take it personally. Rejections feel very personal, because of the personal nature of creative output; this is your head-baby, the product of your imagination, your values and hopes and dreams, and there’s no possible way that isn’t personal. But it’s only personal to you. To the editor, you’re just another name in a slush pile, and they don’t even see you. What they see is the work. What they’re judging is the work. When they reject it, they’re rejecting the work. You, as a complex living person, don’t even enter into it. A rejection of your work is not a statement on your worth as a human being.
And for the love of God, don’t respond to rejections. The only one I ever responded directly to was to ask the editor if I might resubmit the piece if they edited another anthology like the one it was rejected from. And even that might not have been the best idea. As a general rule, just don’t say anything.
3. Take what you can from it. Sometimes, if you’re spectacularly lucky, the rejection you get won’t be a form response. Sometimes it will actually have personal comments enclosed. Treasure these, because they are the next best thing to an acceptance, particularly if the editor has taken the time to tell you what about the piece didn’t work. If this happens, generally it means that the editor liked what they saw enough to want to put in an effort to help your chances the next time you submit somewhere. Or they’re just an absolute patron saint. Either way, if you get critical comments, treat them like the gold they are. Take them to heart. Appreciate them. And if they say they look forward to seeing more from you, in my experience, they mean it. You’re meant to feel encouraged and you should.
It infuriates me when I see people getting butt-hurt about personal rejections. They have no idea what a gift they’ve been given.
4. Get it back out there. Just because one editor has rejected it doesn’t mean all the others will. Hell, even the very next one might not. If you really believe in what you have, if you think it’s good enough for publication, don’t give up. Keep trying. Not only is it good practice for someone trying to get paid for their writing, but I find it has real therapeutic value; I’m less depressed about a rejection if I can immediately roll the dice again and send it to another market. Nothing annoys me more than a story that just sits there. I put work into my writing so that I can make it work for me. This is why I often try to line up at least one or two backup markets for a piece before I send it anywhere.
If you have a story that’s already been rejected more than once, this can be hard to do. But I’ve found that the only way to fight that feeling is to charge it head-on.
That said, there are a couple of stories that I’ve shelved indefinitely, because after a couple of rounds of rejections and an honest stock-taking, I knew they weren’t up to standard. That’s okay, too. If I kept submitting them I’d probably just be wasting my and the slush readers’ time, and God knows they have enough people doing that without me helping. Finally and in any case, I’d rather not have sub-par material out there with my name on it.
5. Give the place that rejected you another try. Especially if they said they want to see more from you. The fact that they rejected you does not mean they hate you or never want to hear from you again. I’ve actually felt awkward about doing this in the past, but I really think there’s no reason to. Especially if it’s a market you really love and really want to be accepted by, don’t give up on them. If it helps, look at it as you generously giving them another chance to recognize your undeniable brilliance and skill. Which is ridiculous, but whatever works.
6. Get back to work. Don’t stop writing. Work on your craft. Work hard. Improve. Giving up because you’re convinced that you suck and you’ll always suck and no one will ever accept you might feel tempting, but it’s a pathetic, sad-sack move. A lot of writers out there really do need to find something else to do with their time, and I suppose there comes a point at which you might need to wonder if that’s you, but in general: go get ’em, tiger.
7. Chill. At the end of the day, it really is not that big a deal, however huge it feels like at the time. Rejections happen. Acceptances happen. But art is a support system for life. Not the other way round.
And just as a parting note, this is probably one of the best posts on rejections I’ve ever seen in my internety travels, and the comments feature this gem, which I hope to God is real:
“We have read your manuscript with boundless delight. If we were to publish your paper, it would be impossible for us to publish any work of lower standard. And as it is unthinkable that in the next thousand years we shall see its equal, we are, to our regret, compelled to return your divine composition, and to beg you a thousand times to overlook our short sight and timidity.” (a rejection from a Chinese economic journal)
A Trick of Light 