Before we delve into the lyrics, let's get some temporal vocabulary:
“Fortnight” means a two-week span. How much damage can two weeks do?
According to this song, a lot. The lyrics describe a torrid, short-lived
and forbidden love affair that, as she says, has dramatic effects: “I love
you and it's ruining my life.”
'Fortnight' Lyrics Meaning: What Is The Taylor Swift Song About?