As a freelancer, you don't have the luxury of a boss telling you that you've done some darn fine work. If you're freelancing at home, you don't get to see the face of someone reading your work so that you can see that you've made them happy with it, or at least that they haven't become angry, turned green and started ripping the place up. All you have is email.
The email can be an important tool for gauging how well you're actually doing at pleasing your clients. When I start to think that my work is basically crap and that there's no reason for me even to apply to a really good gig because no one would ever hire me, I go to my praise file.
The praise file is a collection of emails that I started filing about a year ago when I realized that I was actually doing pretty well and that clients were sending me nice emails about my work. The praise file currently has 30 emails in it, all complimenting stuff that I've written. Ok, one of them is an online compliment of some work that I copied and then emailed to myself, but the rest of them were actually sent to me. It's come in very handy when I need a confidence boost before applying to something or when I think up a new idea and wonder if I'm even good enough implement my own ideas.
If you don't have a praise file- get one. Start archiving those compliments. Pretty soon you'll have a nice email crop growing there to feed you when you're starving for a little resolve.