It's from Chapter 1 of "The Little Foxes" as you correctly suggest:
The bitterest tears shed over graves are for
words left unsaid and deeds left undone. "She
never knew how I loved her." "He never
knew what he was to me." "I always meant
to make more of our friendship." " I did not
know what he was to me till he was gone."
Such words are the poisoned arrows which
cruel Death shoots backward at us from the
door of the sepulchre.
Hope this helps!