I also completely understand the anguish because my daughter is basically tiny me with straight hair and balance.
I think - and remember, I am not a licensed counselor but I am married to one - that as a child I overcompensated for my inattention by being the best at everything. Except sports. I could not catch a ball without injury and was consequently the last one picked for most sports teams. But with school and art and music, sometimes theater, I aimed to excel. I have a distinct memory of sitting on the roof of my house as a teenager working my ass off trying to figure out a chemistry problem. It just wasn't going into my brain. (I sat on the roof for minimal distractions.) Finally, I had to draw electrons and protons to make sense of what I was doing. I didn't throw a tantrum like my daughter, but I did feel like I needed to get it right and was damned sure I wasn't getting off that roof until it happened. Why? Because my entire self-worth revolved around being smart, getting things right.
My daughter has a similar anxiety. She has to get the piano piece right the first time or she falls to pieces. Never mind that rarely does she stop to figure out the notes. She looks at the piece one time and then tries to memorize it. It's a coping mechanism because I think after three years of lessons, she still isn't sure that Every Good Boy Does Fine.
Perfectionism + Anxiety + ADD + Overcompensating + Coping Mechanisms = I'm moving out when my daughter is in high school.