It s like the meanest, wildest monkey running around my head, constantly looking for ways to bite me. That was how Kirsten Pagacz described her OCD to her therapist on their first session when she was well into her 30s she d been following orders from this mean taskmaster for 20 years, without understanding why.

Initially the tapping and counting and cleaning and ordering brought her comfort and structure, two things lacking in her family life. But it never lasted; the loathsome self-talk only intensified, and the rituals she had to perform got more bizarre. By high school she was...