The speaker feels used and cast aside by someone who once adored her. She compares herself to a doll: chosen because she was fascinating, admired while she was new, then thrown away "like every dolls" in the end. The poem sets up a sharp before-and-after pattern with "Then" and "Now." She was once the perfect one, his happiness, his girl. Now she is a "shitty one," his frustration, and finally a "known stranger," someone he knows well but no longer treats as close.
The mood is hurt and bitter, with a quiet sadness underneath. The last two lines carry the most pain. She wishes she had no heart, because a doll can't feel, and then she could have stayed with him forever. The wish shows how much she is suffering: she would rather be an object that is kept than a person who feels the loss. The plain, rough language makes the pain sound raw and honest.