So... Maybe the trick is to find an external measure of success that the person will accept, that validly measures success as well as failure? Could be grades in a challenging class, my DD's medical issues, or the opinions of a respected teacher/coach/etc. Can NOT be grades in a too-easy class, your DD's condition, the opinions of unqualified persons (like those rigid teachers or easily impressed strangers at the supermarket), or self-talk, which can too easily spiral out of control in either direction.

Maybe the really terrifying part of perfectionism is a belief that only the second category exists, which makes ALL success meaningless and all weakness disastrous failure?