Well, I suppose you might be referring to her words to the priest, who had told her to have courage. The translations are varied, but according to this site, she said, "Courage? The moment when my troubles are going to end is not the moment when my courage is going to fail me."
However, these words, assuming they were spoken, are not particularly famous. The words to the executioner whose foot Marie-Antoinette had trod upon, are. As I said in my earlier post, those words were, "Excuse me, sir, I did not mean to do it."
For the more eloquent quotation, see the "execution" section on this site:
https://en.wikipedia.org/wiki/Marie_Antoinette