What is it about witches' prophecies that frustrate Macbeth? Why does he feel his efforts to attain the throne may have been in vain? Does he regard Fate as inevitable?

Macbeth is fighting against what he believes is his fate. The witches have given him the prophecies, which if studied closely reveal exactly what his fate will be. Macbeth's actions are a direct result of his belief that he can change what has been ordained, he might believe in the back of his mind that fate is inevitable, but his actions belie his beliefs.