In order to figure out what the falling action is, it is helpful to identify the story's main conflict and climax. While it is tempting to suggest that the conflict is between the story's narrator and nature, in the form of the bass, it is actually the narrator vs. himself. Knowing how much Sheila Mant hates fishing, how "dumb" she thinks it is, he is completely torn between reeling in the giant bass and ignoring it in order not to betray a love of something she thinks is stupid. He says that it "seemed [he] would be torn apart between longings, split in half." It is his own conflict—between wanting Sheila and the bass—that helps us to understand that the climax is when he "pulled a penknife from [his] pocket and cut the line in half," choosing his longing for Sheila over the fish.
Thus, the falling action consists of his immediate sense of regret: he feels "sick" and "nauseous" when he sees his fishing rod straighten, relieved of its tension. He barely remembers the rest of the night. Sheila whines about her tired legs, she and the narrator go to the fair, and she eventually tells him she's going home with another guy. She tells him that he's a "funny kid," and, in the story's resolution, he reflects on how many people have said something similar to him in the years since and how he "never made the same mistake again" of choosing to please a girl over "these secret, hidden tuggings in the night."