I've seen a few different takes for this song on the sub, and I apologize if this has been said before but I just didn't see it in the sub, here's my interpretation of BTTWS.

BTTWS is definitely a song about grieving a loss, and while it's absolutely written in a way that everyone can apply it's lyrics to their own personal experiences I think something I don't see often enough is the direct connection it has to Would've Could've Should've.

To me personally I see this song as Taylor grieving the loss of the person she feels she was meant to be, the person she could have been had the events of WCS never transpired. Now I'm not here to name a muse for WCS, I'm just here to discuss the connections between these two songs.

WCS is a song about regret, about regretting choices that were made that inevitably lead to Taylor straying off of the path she'd intended for herself. She says that if XYZ.hadn't happened she would have stayed on her knees, she would have followed a righteous path, she would have remained a "good girl"

BTTWS is a song about grieving the loss of possibility, of never getting to meet a certain version of someone "I'm never gonna meet what could've been, Would've been, what Should've been you"

The usage of Would've, Could've and Should've in the lyrics feels very much like an intention nod to WCS, as if she's saying to us "the person I'm mourning is the person that died in WCS" and the person that died in WCS is the potential Taylor she could have been had she not danced with the devil at 19.