Basically, the girl that he was with hurt him. Even though she treated him badly, he still loved her, almost like an addiction. They broke up and he turned to drugs to cover the pain, only burying himself further into the hole. The only one who could mend his hurt is the one who caused him the pain in the first place. He's begging for her to take the pain away. It took something so severe as him dying for her to realize that she misses him.