Ever since Harry and Ginny split up at Dumbledores funeral they new after the war they something had to change, they both had feelings they just didn't have the time or the qualities to be in a relationship right now. Ginny lost her brother in the Battle of Hogwarts and still couldn't quite believe he was dead. Harry was taking all the blame for what had happened at his time at Hogwarts even though everyone scowlded him for saying so. Harry wanted Ginny and Ginny wanted Harry they just had to get their heads round it before they could give it another go.
