DS6 is much the same way and used to throw a fit nearly every time he drew a picture or wrote a sentence at home and at school. We're fortunate in that he had a fantastic teacher who recognized this as perfectionism and emphasized that what she wanted was a "sloppy copy" and that DS could make a perfect copy for himself later on if he wanted to. DS of course never got around to making the perfect copy. At home, we emphasize doing things just "good enough" rather than doing one's best. Just creating a category (good enough) in between perfect and terrible seemed to help a lot. I've also always encouraged DS to draw with pen so he had to learn to live with his mistakes--incorporate them into the picture or just let them go and move on. He's gotten much better in the last year or so and now draws for hours without getting upset.
