In "second best," the speaker admits that rejection isn't really the problem. It stings, but they're used to it. What hurts more is a deeper belief that they have never been good enough for anyone to put them first. They list the roles where this shows up: sister, friend, choice. Each time the word "second" comes before the role, as if it were part of the label.
The middle of the poem shifts from complaint to longing. The speaker wants to be the best friend, the best choice, the best partner, simply to be seen. Then the tone drops. "I'm not worth anything, am I?" is less a real question than a hope that someone will say no. The poem ends on a single word, "Second," left alone on its line, which makes the speaker seem even smaller. The mood is quiet, tired, and painfully honest, with the repetition showing how stuck in this idea the speaker feels.