Follow-up: The night after our little talk, DD actually WANTED to practice guitar, and the practice went wonderfully, with her taking her mistakes in stride. And since then, she has flourished... not only did she learn to play the piece passingly well, but she went and memorized most of it besides. She's also taken to drawing guitars, so she has definitely rediscovered her love for the instrument.

Naturally, DD gave no credit to her mother or myself, but gave it all to this: http://www.apa.org/pubs/magination/441B016.aspx

DW gave that to her the night of our successful practice, so I think we'll go ahead and take credit anyway.

Until the next time the school delivers a major blow to her ego, anyway...