Question

How does ghost undermine the narrators faith in his ability to write ghost stories?

Answer 1

The ghost revealed to the author that it was always she who gave him the inspiration and the plot for her ghost stories. Whenever he needed a ghost story, he invoked heavens for help, in response to which she visited him, stood by him, and made him write the story. This revelation undermined his confidence to write ghost stories. 

Answer 2

Helen the ghost tells the narrator that it was she who had been helping him in writing the ghost stories many a times she had been leaned on his shoulder to give him ideas if she had not help him he would not have been able to write ghost stories does thus the ghost undermines the narrator's faith in his ability to write ghost stories