I'm away at a writing retreat, but here's an older post you might enjoy:
Rejection is a natural part of trying to get your writing published, but that doesn't make it any easier. We're always told not to take rejection personally, but it's almost impossible not to when the project you've been working on for months (maybe years) has been deemed not good enough. I think it's perfectly fine to wallow for a day after that rejection comes in. Yes, it's only one person's opinion but that one opinion can make you feel completely inadequate. But wallowing is only the first step in dealing with rejection.
After feeling sorry for yourself and eating a lot of cookies, you pick yourself up and resubmit your project. If you've gotten several rejections with the same feedback (e.g. your ending needs to be stronger) then you might want to hold off on resubmitting for a few days until you can get that ending fixed. And then you send it out again and again, for as long as it takes, until you find that person who will fall in love with your story.
It may take forever and you may wind up revising a hundred more times and eating a mountain of cookies, but you can't give up! As Brian Yansky so wisely said on his blog, "You have to be stubborn to be a writer." Being stubborn can seem impossible sometimes, especially since writers tend to be sensitive types. But if you've set out to get published then you have to be absolutely bull-headed until you reach that goal.
And in the meantime, while you're submitting and resubmitting, keep writing! If you only focus on one project, you'll limit your potential. What if that first book isn't meant to get you published? What if it's meant to teach you how to write? So be stubborn, keep submitting, but also keep working on new projects, because you never know what will happen, and it'll never happen if you don't keep forging ahead.