If you use the anagram in the preamble to tick off all the uncrossed letters the you should only have two possible letters left. If you can’t form an obvious word, then yes, you may have gone wrong…
It took me a few trial and error combinations of unclued entries in Google to find the quote. I'm surprised they didn't ask entrants, of whom I'm no longer one, to highlight the author just to confirm that they'd all got there.