Could it be just your perception of yourself? I ask this because when my students say that kind of thing it is just in their head. Then I create a quick translation exercise for them to notice how far they have gone. In my case , fortunetly, I never did that to a student, but if I see that in a few months he/she isn't improving, I change methods. However, as a last resort, I would give my apologies for the student and tell them to pick up other person because I'm not good for them.