In W. D. Wetherell's short story "The Bass, the River, and Sheila Mant," what mistake has the narrator vowed to never repeat?

In W. D. Wetherell's “The Bass, the River, and Sheila Mant,” the narrator learns an important lesson about life. As a fourteen-year-old boy with his eye on an attractive, older teenage girl, the narrator has to make a tough choice between the girl and his first true love: fishing.

Wetherell establishes the narrator's (he is not named in the story) love of fishing in the story's first sentence:

There was a summer in my life when the only creature that seemed lovelier to me than a largemouth bass was Sheila Mant.

The writer is using a plot technique called foreshadowing here--the reader gets at least a hint of the idea that there will be some sort of conflict between his love of fishing and his desire to pursue the girl, Sheila Mant.

As the story progresses, we see the narrator struggle to keep up appearances in the canoe with Sheila on their date, while he also tries to surreptitiously keep a tantalizingly large bass hooked on his fishing line. Eventually, he chooses to cut the fish loose:

. . . the tug was too much for me, and quicker than it takes to write down, I pulled a penknife from my pocket and cut the line in half.

“The tug” he is referring to is Sheila's power (derived mostly from her beauty) over him. He gives up the fish to keep Sheila from finding out that he was paying more attention to it than to her. Later, Sheila ditches him for another guy, which leads the narrator to this life lesson at the end of the story:

There would be other Sheila Mants in my life, other fish, and though I came close once or twice, it was these secret, hidden tuggings in the night that claimed me, and I never made the same mistake again.

We shouldn't take this to mean that he will never love someone more than he loves fishing. It's more likely he means he will never sacrifice what is really important to him for something that is just a momentary desire.