The speaker begins by wishing for a safer, kinder world than the one she sees, a place with fewer dangers and less to be afraid of. The first half of the poem is heavy with fear. She is so anxious around other people that she freezes when someone speaks to her, and she hides in the dark so no one can watch her cry. The poem seems to be about bullying, and the final line, "Bully proof," makes that clear.
Then the tone shifts. She speaks directly to someone who "never really cared" and only enjoyed watching her fall apart. The image of a girl made of glass shows how fragile she has felt, but she refuses to stay broken. The mood moves from loneliness and dread to defiance. The ending is short and firm, as if she is trying to convince herself as much as the person who hurt her.