I see a lot of people taking the term “fortnight” literally to meaning a 2 week or short relationship but in the grand scheme of this song, that’s not what its referring to.
The crux of this song is the narrator imagining a future where her and her ex have moved on, it’s been several years and when reflecting back on the relationship and time they spent together, its a fortnight compared to the time they’ve spent apart.
The narrator imagines that in the future she will be having a miserable life as a functioning alcoholic with depression and with a husband who cheats on her meanwhile her ex is having a great life with a very nurturing loving wife. Which she is feeling animosity and jealousy about. “My husband is cheating, I want to kill him” and “Your wife waters flowers, I want to kill her”. You can see the difference in their relationships.
She doesn’t hold much animosity specifically towards her ex though. You can see it from the lines “All of this to say, I hope you’re okay. But you’re the reason. And no one here’s to blame but what about your quiet treason?” She’s saying I hope you’re doing well but the end of our relationship is why I’m in this predicament, you don’t even know but this is why I’ve turned out such a mess.
She imagines her ex and her being on civil terms at this time though seen from the lines “Run into you sometimes, ask about the weather. Now you’re in my backyard, turned into good neighbors”, That doesn’t mean they’re literally neighbors. They’re living separate lives apart from each other but she can peak through some fences to see his life and how he’s doing, likely via social media or friends.
She also reflects on when she tried to get over their relationship when they first broke it off with a “miracle move on-drug but the effects were temporary”. That’s referring to the rebound relationship she had that went no where and didn’t heal her heart.
At the heart of the song, she still loves him and its ruining her life. She has a lot of regret. And reflecting on their relationship it feels like they were together for a short time and she didn’t have him long enough. It was only a fortnight.
Then at the end of the song, we have Post Malone singing and I think he’s singing from the exes perspective when they broke it off. He wanted to reach out to her but in fear that she wouldn’t answer, he didn’t. “Thought of calling ya, but you won’t pick up.”
He thinks he’s giving her the life she wants by letting her go (“move to Florida, buy the car you want”), but she’s actually hurting and misses him. The last line says but the car won’t start until she touches him. Which infers her life won’t be what she wants unless she has him in it.
Relating this song to Taylor’s life – she fears Joe will eventually be happily married to someone else and meanwhile she is doomed to never get her happy ending.