I believe that this song chronicles the before and after of a toxic relationship. When she says "you're growing tired of me," she's alluding to when the relationship became unhappy, and possibly abusive. The line (the most important one, in my opinion) that goes "it's just that I fell in love with a war, and nobody told me it ended" is a metaphor for how, despite the fact that she left the relationship, she's still traumatized by whatever abuse occurred. This is my favorite Mitski song, because it just resonates with me so much.