Don't give up. Seriously. When those rejection letters come pouring in, when you get your first negative feedback, when it seems like no one will listen and you sink deeper and deeper into yourself, take a step back, but only one.
Go for a run, cook a meal, take a class, have a few beers, go on an extended vacation (always works for me). But never give up. Because that's the only way to ensure you won't get anywhere.
And I promise, in a few months, after you've written something else and gone back to edit the story you were subbing, you will see your weaknesses (and strengths, which they never point out in those pesky rejection letters!) and it will be humbling.
And then the next time you put your work on sub, some of the agents will recognize you and some will congratulate you on your progress. Readers will come back and say they like your second book more than the first.
The business is subjective, so don't let one piece of feedback hold you down for too long. We are all waiting for your next work.