Not only is sending a letter to a patient terminating their treatment unprofessional but it's also medically unethical. A decent psychiatrist will discuss the termination process with the patient and assist the patient in preparing for the end of therapy. This should be done in person and not in letter form. If you really feel up to it, you can report the practice to the state's psychiatric board and report what has occurred. It definitely sounds unreasonable that they just dismissed you like a piss ant. Sad that you had to experience this.