In Act 2 scene 2 of Macbeth, how has Lady Macbeth apparently reinforced her courage?

This is a fascinating scene in this excellent tragedy, not
just because it details the events of the murder of Duncan and those after it, but also
because we see Lady Macbeth in a new light. When she enters by herself, we see this
resolute and determined woman apparently doubting the success of the plan that she
concocted and also showing a surprisingly human side as she explains why she was unable
to kill Duncan herself:

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Alack! I am afraid they have
awak'd,

And 'tis not done:--th'attempt and not the
deed

Confounds us.--Hark!--I laid their daggers
ready;

He could not miss 'em.--Had he not
resembled

My father as he slept, I had done't.--My
husband!

However, after this
display of fear, doubt and human weakness, Lady Macbeth is quick to reinvigorate her
former inexorable nature, which seems to be sparked by her husband's reaction to what he
has done. Note the way in which she responds in anger to Macbeth's decision not to go
back to the chamber and smear the grooms with Duncan's
blood:

Infirm
of purpose!

Give me the daggers. The sleeping, and the
dead,

Are but as pictures; 'tis the eye of
childhood

That fears a pinted
devil.

Thus it is that
Macbeth's fear and worry and lack of courage seems to inspire his wife to recover from
her temporary loss of nerve and to complete the task as was planned, thereby framing the
grooms with the murder.