Okay, so the manuscript wasn't quite what the editor wanted. She's asked you to resubmit after you've worked "really hard". I know it's an ego crush to hear your baby isn't good enough.
But think about it this way: no one really gets it right on the first try.
Where would scientists be without experiments? Where would dancers be without practice? Do you really think actors just walk on set and get the scene in one take every time?
There are some out there who think writing is a get rich quick scheme. I hate to break it to you... it's not. Yes, there are writers out there who sell LOTS of books and there are those who make the NYTimes Bestseller list. They have also worked their behinds off to get there. And the book on the list, more than likely isn't their first ever book.
Now I'll be honest, there are some who do strike it rich on the first book. I'd be willing to bet that that book wasn't their first foray into writing. Many writers start out BLOGGING, writing articles for the newspaper, or in freelance journalism. In other words, they wrote before they wrote.
So back to the resubmission... I've been there a few times myself. It is frustrating, but if you get past the frustration and personal pain of rejection, you'll find that the changes are things that will HELP the work.
Are you worried that the editor will change the flavor of your work? Or change your voice? It might, but I doubt it. Your book will sound like the voice you gave it if you stay in command of the manuscript. A good editor will keep what you have built and help you to build on that foundation to make the work stronger. I promise.
Well, enough of my soapbox for today... I need to find some eye candy.