Tangentially related: do you think your mentor might have been indirectly encouraging you by telling you it was impossible? I've been on both ends of that trick and have seen mixed results. Any mentors here who have done something similar? How did it go?
A rebellious student will be challenged by being told they're not allowed to do something.
A prideful or intellectually vain student will be stirred to action by being told something is impossible.
A timid student can be stirred into action by telling them you see greatness in them.
But you really have to get to know them before you decide which method to apply in a subtle matter, as an incorrect choice will be deleterious