In my opinion this song means that the guy was in a long relationship that had a lot of substance and passion. However, his girlfriend kept doing things to hurt him, thus he totally felt "shattered" every time she hurt him. She did something to hurt him, and he tried to leave, but he turned around to go back to her because of his feelings for her. But he kept getting hurt, thus he can't keep turning the car around and going back to her. Thus he's good without her. That's why he does not want to turn the car around, he's got to turn the ("this thing")situation around.