As to his real character, had information been in her power, she had never felt a wish of enquiring. His countenance, voice and manner had established him at once in the possession of every virtue.

– Jane Austen

Pride and Prejudice, Chapter 36. The realization that Elizabeth has been been wrong in her understanding of George Wickham hits home and hard. Because Wickham was charming and handsome, she decided from those first impressions that he must be a good person. So she failed to question the aspersions he cast on Darcy’s character and as a result was prejudiced against Darcy. But Darcy’s letter forces her to reconsider her opinion of Wickham, question her poor judgment of people and realize her lack of knowledge of herself.