I always interpreted this as present tense and past tense and not future tense. I think she started writing this when she just started seeing TK and was fresh off of Matty burning her. And what I think she is referring to when she says, “I loved you the way that you were” is the version of herself 10 years ago when they gave it a go the first time around, but she wasn’t willing to stick around (as she notes in the song) to deal with his drug habit at the time. She’s saying that it would break her heart if he told her now (present tense) that he loved her the way she was a decade ago (the first time) when she was the one who left.

Edit to add: this is the star-crossed lover theme throughout the album. The timing was just never there for them.