I wouldn't equate frustration to disappointment. I see those as two different things. Frustration is pretty normal for all of us, even therapists, but it really isn't a judgment. Disappointment is a judgment.

I've had therapists get frustrated with me for very similar reasons you mention, but I really never sensed that they were actually disappointed. Perhaps that is more about your feelings about yourself than actually what your therapist feels about you. Maybe?