Doctor Reynolds never seems to talk about Bambi's symptoms, but instead her "behavior," and it isn't improving as far as she's concerned. She doesn't think that Bambi is "pliable" or "suggestible" enough to continue her therapy, so when she comes to her room for the day's session she tells her they'll be trying a new treatment: regression therapy.
Regression therapy, she explains, involves moving backward emotionally into a childlike state in order to be more receptive to treatment and to let go of stress. When Bambi sees that her doctor has brought a diaper and baby powder with her, she protests, saying something silly about "patients' rights" which is far too complex of an idea for a little girl like her. So, Bambi is turned across her doctor's lap and spanked, first over her little panties, and then on her bare bottom, until she agrees to wear the diaper.
It's humiliating to be laid back and have her legs spread by the doctor to be powdered and taped into a fluffy, padded diaper, but even worse is the fear that maybe she'll just end up like this forever. Maybe this treatment will turn her into nothing but a babbling baby. Maybe that was the point all along.