chapter 16-18 why elizabeth feels that she has been"blind", "prejudiced", "unreasonable"?

I assume you are referring to tha latter part of the book. Elizabeth and Darcy face internal challenges: their own pride and prejudice. In these final chapters, Austen makes it extraordinarily clear that both characters have changed. Darcy's clear regard for the Gardiners is an external indication of his change, while Elizabeth's new timidity reveals her newfound shame. It is notable that Elizabeth becomes far less active in these final chapters, a shift that leads some critics to observe that the novel's second half is slower than the first. However, the reason for Elizabeth's more tempered demeanor is that she is no longer so quick to jump to conclusions.