The speaker looks in a mirror and sees her younger self, and what follows is a kind of mourning for who she used to be. At first the memory is warm. The girl smiles, laughs and plays, and the speaker admits she envies her freedom, even though she now sees that the worries that felt huge back then were not really stress at all. But when the speaker reaches out to touch the mirror, everything shifts. The girl's smile droops, her laughter turns to quiet sobbing, and she asks "why?" and why she feels so empty and scared.
The poem suggests the speaker has lost something along the way, perhaps her joy or sense of safety, and feels guilty about it. Her whispered "I'm sorry" sounds like an apology to her younger self. The mood moves from nostalgic and tender to sad and heavy. The ending is the quietest part: she wipes her tears, forces a smile, and walks away, hiding her pain from everyone but herself.