What does the author reveal about why Katie feels guilty about her career?

In this part of the novel, Katie begins to focus on Simon's upcoming party causes her to begin having trouble not only in her private life, but in her professional life as well. Katie's meltdown in the classroom finally begins to reveal to the reader what happened between Katie and Matt. For some reason, still unknown to the reader, Matt was unable to attend university and become the zoologist he always wanted to be. However, Katie is living his dreams. This causes Katie to suffer under a great deal of guilt and pressure in her professional life.