In Jane Austen's Pride and Prejudice, what does Elizabeth mean when she says to Jane "and if you lament over him much longer, my heart will be as light as a feather"?
Elizabeth is discussing with her closest confidante and older sister, Jane, what she has newly learned about Mr. Darcy, specifically, that despite what she has believed, Darcy is actually a man of character, while the one who deserves contempt is Wickham. Jane agrees with Elizabeth that Mr. Darcy deserves a second chance, so to speak, and as the young women discuss their shifting opinions of him, it is apparent that they both feel regretful of how Darcy has been treated heretofore. Jane in particular is horrified that anyone could be as duplicitious as Wickham has proven to be, given her enormous faith in the basic goodness of human nature, and is similarly upset about the torment Darcy has quietly endured prior to writing the letter explaining himself. The words referenced in the question are spoken by Elizabeth as the two commiserate, Elizabeth basically saying that Jane feels bad enough about the situation for the both of them.