Darcy completely wins Elizabeth over with his involvement in Lydia's marriage. She is ashamed to think of how much he has done for her family, but she is also deeply grateful for his assistance and is intrigued by his possible motivations. Note that despite the fact that Elizabeth has recognized how well-suited she and Darcy are and that she recognizes his generous and thoughtful nature, she still does not believe he can overcome the detriments of her family, especially now that Wickham is her brother-in-law. At this point, Darcy has proven his willingness to sacrifice a little pride for Elizabeth's happiness, especially in his dealings with Wickham. So perhaps Elizabeth's inability to believe in the magnitude of his love for her stems not from any fault of Darcy's, but rather from Elizabeth's own insecurities regarding her family and her seemingly pointless hopes for a life with Darcy. These insecurities paired with the intensity of her feelings for Darcy cause her to do something extremely uncharacteristic — she does not confide in Jane. Considering that she shares everything with Jane except potentially painful matters concerning Bingley, Elizabeth's silence on such important, life-altering matters is significant and seems to indicate the depth of her uncertainty.
Elizabeth's insecurities are not relieved at all by Darcy's visit to Longbourn with Bingley. His retreat into silence frustrates and confuses her, but instead of attributing his reticence to pride, Elizabeth fairly considers that "perhaps he could not in her mother's presence be what he was before her uncle and aunt."
