I can understand your confusion--this story does have an ambiguous ending.
There is a summary of this story at: http://www.enotes.com/araby/summary. This may help you.
What happens at the end of "Araby" is what James called an "epiphany"--a flash of internal revelation or understanding. After the young boy has revered and almost worshiped Mangan's sister, he overhears a flirtatious and shallow conversation at the bazaar between a young woman and two young men. The narrator tells us that he does not purchase anything for Mangan's sister and that "Gazing up into the darkness I saw myself as a creature driven and derided by vanity; and my eyes burned with anguish and anger."
This moment at the fair has made the boy view his feelings differently--he sees his conversation with Mangan's sister as equally silly and shallow, and realizes that she probably did not take him seriously. It is a "growing-up moment" for this young boy.