I have problems with art block all the time, especially when I try something and get frustrated with how it turns out. But there are ways of dealing with it. You can either wait until you're inspired or force yourself to at least try to crank something out. I try to find inspiration when I can't think it up on my own by checking out challenges, contests, job offers, making fan art, doing requests, coloring line art, collaborating with other artists or participating in projects (like pixel stamps, pixel joint, iso city).
While I'm not making art, I like to study it. I love looking at through pages and pages of art by other artists, trying to figure out how they do what they do, maybe trying out a technique with my own twist. You should never outright copy anything verbatim, but you can't go wrong studying techniques they use, learning about what makes their pieces just right like light sources, anatomy, color theory, etc.
Practice makes perfect... eventually. Even if you don't like what you end up with, you tried. Get c&c and you make it better. And if you get frustrated with something, put it away and try again later.
Anyways, I know how you feel. A lot of artists do. Just keep at it.