I think he's thinking about when he told Sarah on the train his secret which is that he cheated on her. He's also explaining how it was when they were young before he became a jerk. The reason I think he cheated on her was how he became "coarse, cold, & reckless." All ways to describe someone who has not cared about your heart and cheated on the person they're supposed to love and whose heart they're supposed to take care of. Now he can't call her. He broke the relationship. He was "scared" and "young" and so he cheats in order to feel less freaked out about their relationship & make it less important to him. So now he's wondering if it's ever going to be the same.
I think he's thinking about when he told Sarah on the train his secret which is that he cheated on her. He's also explaining how it was when they were young before he became a jerk. The reason I think he cheated on her was how he became "coarse, cold, & reckless." All ways to describe someone who has not cared about your heart and cheated on the person they're supposed to love and whose heart they're supposed to take care of. Now he can't call her. He broke the relationship. He was "scared" and "young" and so he cheats in order to feel less freaked out about their relationship & make it less important to him. So now he's wondering if it's ever going to be the same.