I think something interesting that jumps out from the song, and if you believe I’m wrong please correct me, is that Taylor seems the only one suffering from this what if that’s torturing her brain because Matty is, per her description, doing fine with someone else, and I think this motif jumps out also really strongly in fots imho when she says that she knows she’ll call him once she and Joe break up. I find rather interesting how she’s saying that she took the reign of their entire relationship, at least how they got back together and not how it ended, and Matty basically followed suit
i feel like it's not quite that black and white really. they were both in multiple relationships, they were both (presumably) planning on futures without each other... but it seems (not completely sure, obviously) that once they reconnected, some sparks flew, she was gearing up to leave joe so she indulged it and he indulged it, they both were like... building it up to be this really amazing thing... and matty's little bubble popped first.
i will say, though, that she IS taking the reigns on a lot of stuff but the song i see it in most is i can fix him and ttpd, where she's basically like... yeah, you're self-sabotaging, you're terrible for me, but trust me, i got this, i will make a square peg fit a round hole because it's FATE, and you agreed, it was fate!!! so i can pull the strings to make sure it remains "fated."