Billy had a framed prayer on his office wall which expressed his method for keeping going, even though he was unenthusiastic about living.  A lot of patients who saw the prayer on Billy's wall told him it helped them to keep going, too.  It went like this:

GOD GRANT ME
THE SERENITY TO ACCEPT
THE THINGS I CANNOT CHANGE,
COURAGE
TO CHANGE THE THINGS I CAN,
AND WISDOM ALWAYS
TO TELL THE
DIFFERENCE.

Among the things Billy Pilgrim could not change were the past, the present, and the future.