I find when I'm down on writing I often come out of it after reading in the genre. Go to the library, take out a dozen YA books, and devour them. Pay attention to what works for you and what doesn't.

Another idea is to take a writing class. It's amazing how motivated one gets after paying money for something. You also get to compare where you are in your writing against the other members of the class. An additional bonus is you may find out why various publishers rejected your earlier work. Perhaps you unwittingly made some simple mistakes that are easy to fix.

Here are a few choices for online courses.

Gotham Writers' Workshop

LSSSW: Writing Children's Fiction

Writer's Village University

WritersOnlineWorkshops

Good luck.