I don’t have enough brain cells to understand who/what fortnight is about… the neighbor and husband references?
okay, so like taylor said, fortnight is sort of a prologue to the entire album. it references a lot of other songs (down bad, florida!!!, peter, guilty as sin?) there are a few different ways that it could be read, and i may not stick to this interpretation, but the below is where i landed.
fortnight sets the scene for the entire album and the events that are about to unfold by telling a story about an alternate reality about the future she was headed to in which she and joe ignore their problems, stay together, and build the life they're planning. she's in need of psychiatric help, she's drinking, she's depressed. but the other guy has re-entered her orbit. they're going to keep crossing paths. she feels insane. he's right there. she is insane. her husband cheats on her; she's enraged at him. the other guy is married to someone great; she's enraged at her too. she feels insane. she is insane, because she's fully consumed with the anger of following the bad path instead of taking the risk.
so at the end of the song, she decides she has to run for it. she has to move to america. she has to go to florida and start over. she has to buy the car and let him touch her instead of just daydreaming about it. because if she doesn't scratch the itch, if she doesn't see what happens, if she doesn't chase the guy that's been her "what else?" then well, how will she ever know? and the rest of the record is her realizing that maybe she wishes she hadn't.