The speaker is answering a question people ask all the time, "Are you okay?", and admitting that the honest answer is no. She doesn't want to lie to her friends, and she's glad to have them, yet she keeps up a performance of fake smiles and laughs. Behind it is a sense of loss: she is no longer the "Happy Little Girl" she used to be.
The poem's central image is reality as a grenade going off inside the heart. It suggests something sudden, violent and impossible to take back. She tells her friends they haven't seen her "true self," which sounds half like a warning and half like relief that they've been spared. The last line drops the distance. Reality isn't something she is warning others about anymore; it is tearing her apart now, and she feels powerless to stop it. The mood is raw, lonely and exhausted, with the short, plain lines making the pain feel direct and unguarded.