Not an answer, but just worked with us... re: perfectionism.
As the adults, we've talked aloud about what we're thinking. So, we basically act-out the situation and verbalize it, so child will have something to see/copy (hopefully).
Eg. Draw, make a mistake. Get a little upset, like "oh no!" Then think aloud the process of accepting. Eg. "I will try again" or "I think if I practice this a few more times, I'll get better at it" or "Hmm, look at my funny drawing, ha ha, it was suppose to be a dog, but I think it looks like a bear now. I think I'll make it into a bear instead and draw a cave over here..."
After doing that a few times, I would have similar things happen, then I'll ask child "What do you think?" and let child help me accept it.
We also left things open, undone, unfinished -- at the very beginning to gently counter the demanding internal engine that wants everything "just so". But since then, we don't do that anymore.
Of course, none of the above works for us when child is hungry/tired/over-stimulated.
Good luck! Keep at it
