I think this is about a guy she was in love with, but is now with someone else. (not her absent father, as she's now living with him: "You moved back in with your dad"). He's calling asking for forgiveness for his actions a long time ago, and she needs to go back to her childhood self ("flat-chested, gap-toothed girl") to find the strength to forgive him, as her heart is too full and she's too jaded ("Four chambers just a standard issue // But none had room, forgiveness is huge // And you had two full of ice water // One fulla salt, one packed with coal"). So, she asks her childhood self to trade hearts for the night, so she can forgive him ("Hoping we could trade, just for tonight //like I could borrow your heart // and I could leave you mine"). Despite maybe wanting to get back together, she knows she shouldn't ("you chose this, you know it's supposed to be over"). Ultimately, she forgives him ("So I can call an old friend and I can tell him that we're finally even").
I think this is about a guy she was in love with, but is now with someone else. (not her absent father, as she's now living with him: "You moved back in with your dad"). He's calling asking for forgiveness for his actions a long time ago, and she needs to go back to her childhood self ("flat-chested, gap-toothed girl") to find the strength to forgive him, as her heart is too full and she's too jaded ("Four chambers just a standard issue // But none had room, forgiveness is huge // And you had two full of ice water // One fulla salt, one packed with coal"). So, she asks her childhood self to trade hearts for the night, so she can forgive him ("Hoping we could trade, just for tonight //like I could borrow your heart // and I could leave you mine"). Despite maybe wanting to get back together, she knows she shouldn't ("you chose this, you know it's supposed to be over"). Ultimately, she forgives him ("So I can call an old friend and I can tell him that we're finally even").