Why does lady macbeth fear about her husband after she reads his letter
You will find your answer in Act 1 Scene 5 of the play.
After reading the letter, Lady Macbeth struggles with deep concerns. She fears that her husband won't have the courage to act purposefully. She doubts that he will pursue the kind of ruthless action needed to facilitate his kingly ambitions. Essentially, Lady Macbeth is afraid that Macbeth lacks the necessary aggression to pursue his goals, especially if the actions called for require him to renounce his moral beliefs.
She believes that Macbeth is "too full o' th' milk of human kindness/ To catch the nearest way." Lady Macbeth readily admits that Macbeth is "not without ambition." What is lacking, however, is the will to act without fear of the consequences. So, Lady Macbeth hopes that Macbeth will hurry home. Then, she will be able to wield her influence over him and motivate him to act to secure his position.