Louise is a young woman, or at least a woman looking back on lost time, who is suffering in secret. The poem follows her through the night, when the noise in her head gets loudest and she lies awake crying over "wasted years" slipping away. During the day she hides it all behind a smile, so nobody around her sees how much pain she is in.
The image of a dam slowly filling captures how the feelings keep building, until her tears seem too small to hold them. The speaker sounds helpless and sorry, and the line "It is far too late" suggests a deep sense of hopelessness. The final lines are the darkest. The "noose" suggests she may be thinking about ending her life, and the poem's point is that no one would have guessed.
The mood is heavy, quiet and unsettling. The short, plain lines make the despair feel painfully ordinary, which is exactly what makes it so troubling. It is a reminder to look past a smile.