Perfectionism is usually described as a problem with standards: they are set too high, and the solution is to lower them. Rest more. Be kinder to yourself. Accept that good enough is good enough.
This is advice that does not reach the thing it is trying to address.
The standards are not the source of the difficulty. For most people who experience perfectionism in its more consuming form, the standards are protecting something. The self is organised around the performance. What the person is when the performance is not there is the question the perfectionism is preventing them from having to answer.
This is why reducing the standard tends to produce more anxiety rather than less. The performance is load-bearing. It is not just producing results. It is producing the sense of being acceptable, oriented, worth something. Take it away and there is a gap where a self should be.
The clinical picture is consistent: a person successful by most external measures, who can articulate that they are too hard on themselves, who may have tried CBT or mindfulness or reducing their commitments. None of it reaches the thing underneath. Because the thing underneath is not a cognitive distortion. It is the structure the self was built on.
This does not shift through better self-talk or more realistic goal-setting. It shifts through a different kind of relationship: one in which the person is responded to as a person rather than a performance. In which being present without producing is tolerated, then safe, then something other than a threat to who they are.
The work is not about the standards. It is about what is underneath them.