What is the appropriate treatment for a person who has a seemingly unshakeable obsession, or fixation, with another person?

It’s not exactly a love obsession. Barbara is very strongly attached to her former therapist Catherine, who doesn’t want anything to do with Barbara. (Catherine is probably a mother figure to Barbara.) Barbara is lonely and miserable because she may no longer associate with Catherine, but thinks about her all the time. Therapy has been completely ineffective. It’s as if Barbara needs to be de-programmed, perhaps hypnotized. (The names are changed.)