Mental illness, of course, is not literally a 'thing' - or physical object - and hence it can 'exist' only in the same sort of way in which other theoretical concepts exist.
Psychiatric expert testimony: mendacity masquerading as medicine.
Narcissist: psychoanalytic term for the person who loves himself more than his analyst; considered to be the manifestation of a dire mental disease whose successful treatment depends on the patient learning to love the analyst more and himself less.
There is no such thing as mental illness, hence also no such thing as psychotherapy.
'Psychotherapy' is a private, confidential conversation that has nothing to do with illness, medicine, or healing.
Individual psychotherapy - that is, engaging a distressed fellow human in a disciplined conversation and human relationship - requires that the therapist have the proper temperament and philosophy of life for such work. By that I mean that the therapist must be patient, modest, and a perceptive listener, rather than a talker and advice-giver.
No further evidence is needed to show that 'mental illness' is not the name of a biological condition whose nature awaits to be elucidated, but is the name of a concept whose purpose is to obscure the obvious.