Knowing the theory is a small part of it. Dealing with irrational patients is the main part. For example, you could go to therapy and be successful. Five years later something could happen and you face a reoccurrence of the issue. It is very difficult to just apply the theory that you already know again. You're probably irrational. A therapist prodding you in the right direction and encouraging you in the right way is just as important as the theory.