"I bear a charmed life"- Who is the speaker?What was the 'charmed life' refer to? How is the claim proved false?

This line quoted from Act5 Sc8 of Macbeth by william shakespeare is spoken by Macbeth.

               Here 'charmed life' refers to the life of Macbeth. Macbeth thinks that his life is protected by a magical power as none born of woman can do him any harm: "The power of man, for none of woman born/ Shall harm Macbeth."

                  But Macduff, to Macbeth's great horror and dismay, tells him to give up his faith in such a charm as Macduff is not born in a natural way. Infact, he was prematurely taken out of his mother's womb: "Macduff was from his mother's womb untimely rip'd." Thus Macbeth's all hopes go to doom. He has lost all hopes of survival. His faith shaken, he is too broken to rise to the challenge.He is no more invulnerable than any other man.