Rejection is a special kind of "This sucks! I suck! Everything sucks!" In this case, your brain, which is still being a douche, has evidence to point to. Exhibit A: this agent doesn't like you. Exhibit B: this other agent doesn't like you. Exhibit C-Z + 20: These agents don't like you. (Not to mention reviews. Not to mention AMAZON reviews. Not to mention the comments section.)
Know that everyone gets rejected. EVERYONE. Read any author blog, listen to any author interview, no matter how good they are, they have faced rejection. A few years ago, I went to a signing for one of my favorite authors, Jasper Fforde. One of the things I took from that was his experience getting published. He wrote a book, one of my favorites of his, The Big Over Easy, a nursery rhyme mystery about the murder of Humpty Dumpty. It was not published. He wrote a sequel to that about Goldilocks and the Three Bears. It was not published. Then he wrote The Eyre Affair. It was published...but it took YEARS. Eventually, obviously, all the rest of the books were published, but it shows the kind of persistence and patience you need to have.
Before Inkshares and The Sword and Laser contest, I tried to get my book published traditionally. A book I read (I'm too lazy to go find it) said to make a list of 50 agents before you start and start sending out queries 5 or 10 at a time. I did 20-25. All were rejections. Then I did a complete rewrite and another 20-25. Again...all rejected. The last two years, I've submitted scripts to WildClaw Theatre's Deathscribe competition. After my first go, I spent an entire year on a single idea. I put everything I had into this script. I believed it was the best thing I had written, the culmination of all the work and lessons learned over the course of my writing career. My absolute A game. And I still didn't make it in. I was fairly devastated. I vowed I wouldn't submit again. After all, what was the point? If something I spent a year thinking about and working on didn't get in, what chance does anything else have? I stormed off in a huff.
Nearly a year and the faded memory of that pain later, I just submitted my second script to Deathscribe. The culmination of all the work and lessons learned over the course of my writing career. My absolute A game.
Basically, it's gonna suck. And be upsetting. And you're going to complain and cry and rage...