Right at the start of the story, Lupe's accomplishments are impressive. But all Lupe seems to notice is that she cannot run as fast as the other girls in school.
Another example of Lupe comparing herself to others is when she describes her trouble with team sports. The text clearly says that Lupe was "no good at sports." Lupe seems embarrassed that she isn't the star player of the team.
Lupe also feels badly that she only recently learned to ride a bike, could only walk in a swimming pool, and rollerblade if her dad held her hand. This shows that Lupe is frustrated that she isn't the best at everything.
As Lupe began to think about these things, she had the idea of learning to play marbles. She made the decision to teach herself to play and began feeling hopeful about being good at something which she can compete against others. This reminds me of myself when I am feeling out of practice in something. I compare myself to other people and feel insecure. But when I focus on my positive qualities, I find that I can do a good job. I hope Lupe keeps up her good attitude and feels proud of herself soon.