“That still doesn’t mean you can do anything.”

Give Me an example.

I started to describe the case of a woman I know who has very strong convictions and great force of will, and yet often fails to achieve her goals.

No, (give Me an instance) from your own life.

“Just winning a tennis game, for example.”

Give me a break.  (A) You always have mixed thoughts in those situations and (B) I said you can’t alter physical laws.  If you completely wanted to win at tennis and believed you could, you would practice, exercise, and so forth.  When I say you can do anything, I don’t mean that you don’t have to take the necessary steps.  Napoleon was charismatic but he still had to train troops, plan logistics, and so on.  Stop being simple-minded.  You are fixating on a single meaning of “you can do anything” and trying to rebut it.  Instead, think about what meaning could be true.  It certainly does not mean wish-fulfillment.  Think about it.