The degree of personality change that can be effected through psychotherapy depends on: the talent and knowledge of the therapist; the degree of early-life emotional damage that the patient endured; and, to a degree, their intelligence. But luck is a factor too, as with all activities. Ideal mates may meet through chance or not, and one may carelessly cross a street. Consider our world had Winston Churchill been fatally injured upon being hit by a car while crossing a New York City street in 1931. But granted the best of fate, the question remains: how much change can psychotherapy effect? A great deal but not coomplete.
The critical ego capacities develop early in life. These enable the child and later the adult to control their thinking and behavior, distinguish reality from fantasy, modulate their mood, and create a sense of who they are or, as psychologists term it, a "sense of self." But for these to develop the child must experience a "good-enough" parenting. Which depends on luck since even the best intentioned parents are a product of their own life experience, babies are not born with instructions, and good health services and nutrition are not available to all.
The therapist's task is to provide the equivalent of the "good-enough" parenting which the patient lacked early in life. Then, like a plant yielding toward the sun, the mind becomes nourished to heal the past psychological damage. Yet even as some plants that receive sufficient nutrients fail to flourish, all therapy is not successful. It takes a long time for an infant's mind to become adult, the early mental structure is its bedrock and, being conservative like all nature, resists change, aided by its powerful unconscious which demands respect.