Isn’t doubt a feeling, too? “A feeling of doubt” is a common phrase.
All I am trying to convey here is that a bit of empathy can go a long way. If someone says they feel a certain way and are having a hard time with it then at least consider the possibility that this is not a switch they can just flip.
I would also suggest that it would really help to focus on the process instead of the outcome. To use an analogy, scolding children for getting bad grades will probably not have the same success as talking to them, learning how they learn, and then finding a learning strategy that works for them. Focusing on the process of learning can lead someone to a love of learning and a love of knowledge.