L.H.
My 6 year old has those tendencies too, and honestly, the thing that really made a breakthrough for her was the Wii fitness games. I'm not a big fan of video games, but this really got the ball moving for us on this issue.
What happened was my husband got her playing the bowling game, and we of course had tears and "I quit" right and left. But as we worked with her, we tried to emphasize that we were playing for fun, not to win or do it perfectly. We showed her that we got plenty of gutter balls, just like her, and we talked to her about how practicing helps us to get better. When she got gutter balls, we'd laugh and say "Oh well, no big deal." Gradually, she learned to laugh at herself and eventually got better.
A video game was what opened the door for us, but now we can refer back to that when she gets frustrated and she understands what we're talking about. And you could do this with anything--sports, music lessons, games. I think you just need something that they think is really fun to keep them interested even when they aren't "perfect," and then you can use it to teach this lesson. As for the writing, I would back off and let her decide when she's ready.