What specific events in the play of Macbeth show an individual suffering from despair?

In Act V, Scene 1 of William Shakespeare's
Macbeth, secure in his castle at Dunsinane, Macbeth prepares for
battle.  His servant enters and tells Macbeth of the multitudes of the English force
that march toward the castle.  Macbeth berates the servant for his cowardice; however,
his bravado conceals his inner despair.  In a declaration that the forthcoming attack
will either give him the throne or kill him, there is evident in his speech the shadows
of his increasing despair:

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Seyton!--I am sick at
heart.

When I behold--Seyton, I say!--This
push

Will cheer me ever, or disseat me
now.

I have lived long enough.  My way of
life

Is fall'n into the sear, the yellow
leaf,

And that which should accompany of old
age,

As honor, love, obedience, troops of
friends,

I must not look to have; but, in their
stead,

Curses no loud but deep, mouth-honor,
breath,

Which the poor heart would fain deny, and dare not.
(5.3.21-31) 

This despair
reaches a crescendo when Macbeth learns that his wife has died.  Like the melancholic
Hamlet, Macbeth, in a brown study, reflects on the brevity of life and its existential
absurdity:

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Tomorrow and tomorroe, and
tomorrow

Creeps in this petty pace from day to
day,...

And all our yeterdays have lighted
fools

The way to dusty
death....

Life's but a walking shadow, a poor
player

That struts and frets his hour uon the
stage

And then is heard no more.  It is a
tale

Told by an idiot, full of sound and
fury

Signifying nothing.
(5.5.19-30)

As though in
defiance of this despair, Macbeth yet displays his courage and heads into battle; ever
the admirable warrior, Macbeth declares bravely, "I will not
yield."