In Jane Austen's Pride and Prejudice, the biggest changes in Elizabeth's character begin taking shape in Ch. 36, or Ch. 13, Vol. 2, immediately after she reads Darcy's letter to her. The first thing she realizes is that Wickham had lied to her. Darcy, like his father wanted, had offered Wickham the parish (church) on the Pemberley estate so that Wickham could serve as the clergyman. Wickham refused it and instead asked for money to go to law school. Wickham carelessly spent all the money and then seduced Darcy's sister, trying to get her to elope with him so that he would own Miss Darcy's fortune. Elizabeth knew that Darcy's version of the story was the true one because he would never tell anyone his sister had been violated unless it were really true. She also knew Darcy was telling the truth because Mr. Bingley would never say that Darcy was innocent unless it was really true.
Elizabeth's beliefs that Darcy is a despicable (hateful) man rested on two things" 1) That he was proud; 2) That he had stolen a living from Mr. Wickham left to Wickham by the late Mr. Darcy (Mr. Darcy's father). Now that she finally sees that Wickham lied to her and that Darcy is really innocent, she realizes that she actually has no reason to hate Darcy. She feels ashamed and realizes that she was prejudiced against Darcy.
Her revelation that she was prejudiced against Darcy helps her to see that Darcy could also not be blamed in breaking up Jane and Bingley. Darcy says that one reason is because he frequently saw her father, mother, and younger sisters acting with rude manners, especially at the Netherfield ball. His second reason is that he did not really believe that Jane was in love because she did not show it. Her manners were too reserved. Even Charlotte warned her that Jane needed to show more feeling, and now Elizabeth is forced to agree.
Hence Elizabeth can no longer see Darcy as proud or horrible because she now realizes that he was innocent with respect to Wickham, that her family behaves poorly, and that Darcy couldn't be blamed in breaking up Jane and Bingley.