I was watching an interview with Tegan Quin, and she was explaining the song call it off. 

She said that the line "maybe I would have been something you'd be good at." was really upsetting to her- because she felt that she had been putting so much into a relationship that wasn't really going anywhere. It was a moment of realization for her that this relationship wasn't going to go anywhere and how upsetting it was. And almost of a way for her to let go of it.