There are several instances of Miss Emily's delusions about reality. One is her refusal to pay taxes, because no one in her family has ever paid. And she insists on this, even though the aldermen send repeated notes and letters to her home, and make a personal visit. Reality--a group of men, telling her to her face--was evident, but Miss Emily clung to her preconceived notions and insisted on casting her perspective of the world on other people. The original reason her family didn't have to pay taxes was even a made up tale that no one really believed:
"Colonel Sartoris invented an involved tale to the effect that Miss Emily's father had loaned money to the town, which the town, as a matter of business, preferred this way of repaying. Only a man of Colonel Sartoris' generation and thought could have invented it, and only a woman could have believed it."
So, Colonel Sartoris invents a fictional tale about her taxes, and Faulkner himself writes that "only a woman" would believe such a falsity. But, Emily does, even when the elders tell her it's wrong.
Another example of her blurring the lines between reality and fiction is after her father dies. She keeps the body in the house for three days, refusing to even admit that he had died. In fact, when people come to help her out, she answers the door and told them "that her father was not dead." She finally breaks down and lets them take the body away, but it took a while.
And finally, the last and most horrific example is that of Homer Barron's body in her bed upstairs. She obviously blurred the lines of reality there; he probably told her that he didn't want to be with her. She didn't accept that. She made an alternate reality for herself where he could be with her, forever. She ignored the fact that he wasn't alive or real, and that his "love" for her wasn't there, and forced the situation to meet her deluded visions.
I hope that those thoughts help; good luck!