Teresa has been told that her course of therapy is likely to take a year or more because it involves the reshaping of her personality and that takes many sessions. her therapy is most likely:

Psychodynamic therapy

Psychodynamic therapy involves uncovering unconscious disturbances and processes that shape our behavior, preferences and habits. Once these processes are uncovered and accepted by the client,  the client is then provided with tools to resolve their issues and overcome negative habits. This process is quite long and can end up reshaping a client's personality if the therapy is provided consistently and effectively. 

The therapy which Teresa is undertaking is likely a form of cognitive behavioral therapy. In this type of treatment, the patient and therapist work together to reshape the thought processes of the mind and the perceptions of self, and to address the underlying issues that may be producing the symptoms or disorder which has been seen in the patient. As such, this therapy takes many sessions over a long duration.