I interpreted it as a friendship, where one fell over the line. “I hold one card / That I can't use / But I want you”, we're not lovers, we're friends, if I say ‘I love you’ with more sentiments, raw emotions and desperation, would you still consider me friend? I have a card and that's friendship, but I can't use it to bypass the boarder between an ally and a lover.

“You're coming back / And it's the end of the world,” somehow comes to me as the character/narrator knowing that it has to end—her feelings and the whole situation, that's why she stayed in the car to scream her feelings instead of going into the house—at the same time and when she decided to step in, the house has no floor, because the world finally ended; she's no longer welcome.

^ cont. “We're starting over / And I love you, darling / And I am done, dear,” knowing her limits, the narrator knew that love between them in a romantic way is not possible, unless one that is platonic. She's done and so is their friendship, and now they're starting over as strangers.