Um...I don't get why you just don't mail the letter? Send it certified mail if you want to be sure she got it. I don't see why it has to be in person, in whatever context - public event, showing up at her office. And I frankly would not pay a therapist who hurt me for an appointment. (And I also don't think anyone can predict how she'll react.)

But I DO think you should express your experience and wish you luck.