I think that the couple broke up maybe he cheated on her but their love never went away so she is seeing another dude and loves them both to much to just chose one so she wants her ex to share her with her new guyfriend but she bleeding inside from not being able to choose so she calls him and tells him he stabbed her in the back he doesn't want to share her but it was his mistake first so he can't get over her but she decides the other guy cause he walked all over her so at the end he gets his heart broken and he sings the stab my back to her like she did to him in the first part like the role changed and he felt what he put her through.