You would greatly benefit from analyzing Act V in order to answer your question. Key to thinking about your question is the way that Macbeth takes false security in the prophecies of the witches that he receives in Act IV. The prophecies they give him are interpreted by Macbeth as surety of his invincibility, and thus again and again in Act V, no matter how many of his lords and soldiers desert him, Macbeth clings to the prophecies and uses them to expel his own fear. It is, however, when the woods seemingly appear to be walking towards his castle that he begins to doubt the prophecies he has placed such faith in:
I pull in resolution, and begin
To doubt the equivocation of the fiend
That lies like truth: 'Fear not, till Birnam wood
Do come to Dunsinane:' and now a wood
Comes toward Dunsinane.
In a similar way, even after this, he holds fast to the promise that he can only be killed by somebody who was not "of woman borne," and as a result wildly and rashly defies the attackers until he realizes that Macduff was not borne of woman. We can see how his overconfidence directly relates to his faith in the prophecies that he received.