Bob refuses to discuss his adolescent years with his therapist. he makes up excuses for not doing so by saying, "i don't remember what happened during those years, it must not be important." what is this tendency called?

I believe the answer is: resistance

In therapy, refers to the unwillingness from a patient to follow the full program and make effort to change their situation.
Resistance is very common on the initial period of the therapy, but it tend to start diminishing overtime after they felt the positive results that created because of the therapy.