Am only a few listens in, so may be misinterpreting the song, but here goes.
In my head, it is a man who lost someone he loved, and is planning on committing suicide to join her, and is very close to actually going through with the act, but then decides not to for her, as he promised her he would stay. They’d spoken about what they should do without each other, but he’s not prepared for that happening, and so is struggling, as you would.
But that last verse, it is him coming back from that edge.
It’s just like you tell,
It is the smell.
I’ll stop, I hope you’re feeling well.
And if you feel the need, I hope you’ll give me a yell.
Always I will hate these sudden things.
He’s that close to dying, he can smell it - and there’s the insinuation she knew she was dying or going to. But he stays alive for her, but if she needs him, he will always be there. For most the song, he’s saying goodnight to her so he can join her, but he instead he lives, and says goodnight to her every night as he probably did when she was alive and they were together.
Tis my take from it, but feel I may be going too literal. But then some Cosmo songs are literal, and incredibly earnest in his playing of that, because it’s not a floaty romanticism of a situation, it’s a ‘this is it, and it feels like this and that’s just how it is, and this is a take of it’ (e.g. Problems, Is The World Strange?, We Just Wanna Talk, etc.)
Hey, where have you been?
And what’s my sin?
Well no balloon and careless fin
Trying to do exactly what you keep asking.
And I pray you are the kind to change your mind.
Many times, first time when I was 13 or 14, and then at least once a year since then, probs about 12 times in total.