The book that I have says in the source part:

"Note Bares Twain Attempt at Suicide," Los Angeles _Times_ , April 15,
1951, I,40.

The page before the quote says:

The treachery which Clemens finally convinced himself was true
depressed him to the point that he recollectd an earlier attempt
at suicide.  Two days after Ashcroft left for the last time, the
shaken master of Stormfield

"called to my mind _my_ experience of 1866 and I
  told it at dinner.  I put the pistol to my head but
  wasn't man enough to pull the trigger.
    Many times I have been sorry I did not suc-
 ceed but I was never ashamed of having tried.
 Suicide is the only really sane thing the young or
 old ever do in this life."

Funereal brooding was perhaps not mere coincidence.

Jules