Perfectionism is characterized by beliefs and feelings that drive individuals to pursue unattainable standards of excellence and impeccability, often at the expense of their psychological and physical well-being. It correlates with many psychopathologies and a heightened suicide risk; and it can be also an aspect of a multiplicity of personality disorders. Perfectionism seems a common phenomenon among psychological practitioners too, and the very few studies that focus on it overall suggest that perfectionism in therapists is negatively correlated with therapy effectiveness, and positively correlated with therapist emotional distress (e.g., negative emotions toward patient characteristics or demands); but it is not clear the mechanism underlying these correlations. In this paper we present a hypothesis on the process underlying the relationship between therapist perfectionism, therapist emotional distress, and ineffectiveness of therapeutic interventions. Starting with a parallel analysis of a clinical case and supervision over it, we show that perfectionism may be part of an overall organization of the meaning of the experience that the therapist brings into the relationship, which we call Prototypical Adaptive Modality of Existence (PAME). We show how the therapist’s perfectionist PAME functions as an automatic process that hinders intersubjective attunement with the patient; finally, we show how a supervisory process focused on the shared exploration of the therapist’s perfectionist PAME, of its developmental genesis, and of its impact on the supervisory relationship can modulate the therapist’s perfectionism, foster attunement with the patient, functionally modify the line of intervention, and foster a positive outcome.