The speaker feels overlooked and undervalued. She opens with a sharp comparison: she is like a rental car, something people use but never choose first. From there she piles up similar images, the "second thought" and the "third wheel," to show how easily she gets left out. Her successes go unnoticed, yet her mistakes are remembered. Even classmates who pair up with her on projects do it because she's smart, not because they like her.
The mood is tired and quietly hurt. Repeating "I'm that girl" makes the poem sound like a list of complaints she has carried for a long time, and she admits she's "tired of it all." She mentions thinking about moving back, which suggests she feels she doesn't belong where she is now. The ending is the most painful part: she cares with her whole heart, but that care is "thrown back at her." It leaves us with a girl who gives a lot and wants to be valued in return.