What were Elizabeth's supposed feelings and thoughts when Mr.Darcy insulted her at the Meryton Ball in Pride and Prejudice?
Jane Austen's Pride and Prejudice is about exactly those two things, pride and prejudice, and both of them are evident in the incident at the ball to which you refer. In fairness to Mr. Darcy, he did not speak his unkind words directly to Elizabeth; that would have been unbearable and we cannot be sure exactly what Elizabeth's response would have been. She simply overheard them, and Darcy's words are what shape the rest of the novel.
Elizabeth certainly felt angry about being insulted, as anyone would, at being called too plain or not beautiful. It is not as if she is unaware that she is not the most beautiful girl on the room or her family is poor or that her mother is a bit (okay, a lot) too forward in her efforts to make good matches for her daughters or that she has several sisters who are silly and foolish. However, it is not the right of a perfect stranger to speak about such things, and Darcy's words would cause most people to become defensive.
In addition to feeling angry and defensive, I suppose I might also feel hurt, as Elizabeth does. I am certain Darcy regrets making such an insensitive and arrogant comment because the emotions it stirs up in Elizabeth keep her from wanting anything to do with him for a very long time. Eventually Elizabeth relaxes her pride and Darcy disavows his prejudice, but those careless words overheard at a party create a lot of misery for them both.