I've been there. When my agent was subbing my first book, I edited my second book and wrote my third book, which really helped. Writing the third book was pretty much the only thing that kept me sane. But now that my agent has told me she's not really interested in the third book, I'm right there with ya. I know I should move on and write a whole different book, but blaaarrrggghhhh, this whole thing's so discouraging. The thought of starting a whole new book and going through the shit of figuring out where the plot goes is emotionally exhausting. So no words of wisdom here for you, I'm afraid, just lotsa hugs.