Imajar is correct, this isn't about heroin addiction. Think a little deeper: What exactly does he want "shot in his arm." Yes it conjures images of drugs, but what he wants is some kind of substance injected into his system that will allow him to love this woman again. She has changed from her previous self. He cannot love this changed person without help from some fantasy injection. The first stanzas simply articulate how this woman feels terrible about what has happened.
Imajar is correct, this isn't about heroin addiction. Think a little deeper: What exactly does he want "shot in his arm." Yes it conjures images of drugs, but what he wants is some kind of substance injected into his system that will allow him to love this woman again. She has changed from her previous self. He cannot love this changed person without help from some fantasy injection. The first stanzas simply articulate how this woman feels terrible about what has happened.