The speaker looks at an old photograph and is struck by how much has changed. The girl in the picture had a breathtaking smile, big dreams and a shy streak that sent her hiding behind her mum. The person she has become rarely smiles, and the speaker senses that even the things that do make her smile don't reflect who she really is.
The poem is written to "you," which makes it feel like a private address, though it's unclear whether the speaker is talking to a friend, a sibling, or an earlier version of herself. The poem doesn't say. Either way, the feeling is a mix of grief and tenderness. The speaker misses that confident child, apologises for not being able to shield her from a world that broke her heart, and ends with a simple wish. The mood is quiet, wistful and sad, but the final line, "You deserve to smile again," leaves a small note of hope and reassurance.