Student or Learner
It is obvious that you cannot be too diligent.
= It is better for you to become more diligent ?
= you are too diligent so you cannot be that ?
Can't "..... cannot....too...." mean it is better to do more and more?
I would use it if I wanted to say that it is always better to be more diligent, so there is no point at which more diligence becomes a bad thing. In other words: I would mean that the adverb 'too' can't be used to modify the the adjective 'diligent'.
Absolutely not a teacher.
From diligence you can't have enough.