I remember watching a lot of horror/suspense anthology shows in the 90s but one I can't remember sticks out in my mind prominently.
In the episode a woman is getting help from a psychologist to block out an alternate personality that exists inside of her mind (which is represented physically in the show). The psychologist teaches her to build a wall inside her mind to trap the alternate personality but ultimately the strategy backfires and the, for want of a better term, "good personality" is trapped behind the wall instead.
I believe the show was in color.
Unfortunately Google keeps turning up actual cases of mental disorders when I try to find this so I am hoping some other avid-watcher of anthology shows recognizes this description and can help identify the show/episode.