Macbeth is fearful of the witches' prophecies due to the fact that they all have come true. He knows that Macduff will return to strike his revenge which will leave Macbeth powerless, dead and losing the position of King he sacrificed so much for, which petrifies him.
"Macbeth! Macbeth! Macbeth! Beware Macduff!/ Beware the Thane of Fife! Dismiss me. Enough" (Shakespeare 4.1.81-82).
"Tomorrow and tomorrow and tomorrow/ Creeps in this petty pace from day to day/ to the last syllable of recorded time,/ And all our yesterdays have lighted fools the way to dusty death. Out, out, brief candle!" (Shakespeare 5.5.22-26).
Macbeth realized after Lady Macbeth's death that people are bound to die, which makes life meaningless in a way of actions. Actions or things that happen in life do not matter because they all eventually lead to death. He realized that death awaits him in the end which lets him know that the guilt pent up in him will be released once and for all
Resolution
Macbeth after killing King Duncan gained a fear of getting caught. He lost sleep because of the weight of his own actions, which made him lose peacefulness and a sense of calmness. He is now scared that people will find out the truth about his devious and greedy actions.
"Methouht I heard a voice cry "Sleep no more!/ Macbeth does murder sleep" the innocent sleep,/ Sleep that knits up the raveled sleave of care,/ The death of each day's life, sore labor's bath,/Balm of hurt minds, great nature's second course,/ Cheif nourisher in life's feast" (Shakespeare 2.2.47-52).