Hm. I guess... just keep trying. The best thing to do is to pace yourself. Take a break and do something else like playing a game or listen to music. Then, if you can, go back to it and see if you can come up with anything. ^^ I know if I overdo it, I end up getting more frustrated.
Well... it's not easy. O_o I don't really know how to explain it. Most of the time, I have to be interested in something. Hm. Well, maybe try finding some inspiration? That's what I do.