Why does elizabeth blame herself in part for Lydia's troubles?

Answer 1

Elizabeth thinks she could have prevented it by being open to her sisters about Mr. Wickham's true character. She knew he was no good, but did not tell the rest of her family.

Answer 2

Elizabeth blames herself in part for Lydia's troubles because she believes she should have done more to protect her sister from making impulsive decisions. She feels responsible for not providing enough guidance and support to prevent Lydia from running off with Wickham. Elizabeth also regrets underestimating Wickham's deceitful character and not warning Lydia about the consequences of her actions.