Despite the burgeoning technologies in the field of "helping", on many levels psychotherapy is still a crapshoot. Some of the goal of training, I think, is to help students accept that fact. The work is part science, part art, and part luck. Learning to tolerate the anxiety inherent in that recipe is critical for any clinician.
Martha Manning, in Undercurrents (1st edition ed.). HarperCollins. 1995. pp. p. 9