The speaker misses a boy who once made her feel special, as if she mattered. Then he gets a girlfriend, and the poem turns on that small, blunt word "But." She watches, or imagines, him kissing and hugging this other girl and making her happy, and she wants that same happiness for herself.
What hurts most is where her thoughts end up. She doesn't blame him or the girlfriend. She decides the problem is her: she doesn't deserve to be happy, he is "so perfect," and "no one wants me." One painful moment of jealousy grows into a verdict on her whole worth.
The mood is sad, raw, and very honest. The short lines and plain wording feel like thoughts spilling out without polish, which makes the self-doubt feel real. It captures how heartbreak can make someone feel invisible.