A key aspect of this tremendous novel is the importance of first impressions and their innaccuracy. So important is it in fact that Austen herself originally planned to call this novel First Impressions, until she later changed it to Pride and Prejudice. This should help us to focus on the way in which both we and other characters are taken in by first impressions, which sometimes may be accurate (as in the case of the stupid and odious Mr. Collins, but are more often than not shown to be innaccurate, such as in the case of both Mr. Darcy and Mr. Wickham. Note that these two examples show that it is dangerous to assume that somebody's character is bad or good based on first impressions.
An absolutely essential chapter that helps establish this theme and also shows a key moment of self-realisation is in Chapter 36, which is when Lizzie reads the letter that Mr. Darcy sends her detailing the truth about Wickham and what he has done in the past. This forces Lizzie to confront the way in which she has been blinded by first impressions. Note the following quote from her:
But vanity, not love, has been my folly. Pleased with the preference of one, and offended by the neglect of teh other, on the very beginning of our acquaintance I have courted prepossession and ignorance, and driven reason away, where either were concerned. Till this moment I never knew myself.
So Lizzie realises that she has been very "blind, partial, prejudiced and absurd" in her relations with both men. However, going back to your statement, what is key to realise is that Lizzie is not trapped by her first impressions. Following her moment of insight, she is able to let what she has learnt about these two characters shape her interactions with them. Thus she softens up towards Mr. Darcy, and becomes suitably distant from Mr. Wickham. Although the novel shows the importance of first impressions, it also rather encouragingly shows the possibility of how first impressions are not always our last impressions, and how we can change our ideas about people if we are humble enough to admit our mistakes as Lizzie is.