One of these days is Yesterday with all its mistakes and cares,
  its faults and blunders, its aches and pains.

Yesterday has passed forever beyond our control.
  All the money in the world cannot bring back Yesterday.

We cannot undo a single act we performed;
  we cannot erase a single word we said.
Yesterday is gone forever.

The other day we should not worry about is Tomorrow
  with all its possible adversities, its burdens,
its large promise and its poor performance;
  Tomorrow is also beyond our immediate control.

Tomorrow's sun will rise,
  either in splendor or behind a mask of clouds, but it will rise.
Until it does, we have no stake in Tomorrow,
  for it is yet to be born.

This leaves only one day, Today.
  Any person can fight the battle of just one day.
It is when you and I add the burdens of those two awful eternities
  Yesterday and Tomorrow that we break down.

It is not the experience of Today that drives a person mad,
  it is the remorse or bitterness of something which happened Yesterday and the dread of what Tomorrow may bring.

  Let us, therefore, Live but one day at a time.

Author Unknown