What change does the antagonist make in "Pigeon Humor" (from the book Sixteen)?

When we think of an antagonist within a story, it's easy to picture a villain who opposes the main character and stands in the way of them accomplishing their goal. However, not every story has a human villain standing in their way. Sometimes an antagonist is simply a force that the protagonist must contend with.
This is largely the case in "Pigeon Humor" by Susan Beth Pfeffer. At first glance, one might think that the main character's mother is the antagonist of the story, as she is a caustic and bitter woman who makes biting remarks throughout the tale about the main character's father, who has just died.
However, the mother does not oppose the character. Rather, the things opposing the character are the recent death of her father and, more importantly, her own emotional response in dealing with her father's death.
The main character, Tracy, had a troubled relationship with her father. She continually thinks about the fact that her father has just died, but she is having a difficult time coming to terms with her emotions.
This emotional blockage is the primary antagonistic force in the story. One can really see this by examining the change that takes place in the ultimate scene, during Rachel's concert. While Tracy's emotions have been so subdued that she has been unable to make sense of what is happening to her or her own feelings, at the concert she is so overcome by amusement—or what seems to be amusement—that she bursts out into laughter.
This laughter is punctuated by tears, though, revealing that Tracy has broken through the emotional blockage that has been hanging over her. Rather than continuing on with dulled feelings, Tracy feels everything that has happened to her all at once in this final scene.
Therefore, one can see that the antagonistic force—the emotional blockage—is something that breaks down over the course of the story, allowing Tracy to begin the grieving process after her father's death.
https://www.roanestate.edu/owl/elementslit.html