Complex sentence: Initially they were writing a sad ballad about a girl that he had traveled around the southern United States for but who broke his heart, but Harmon decided to change it to an upbeat song, and said: Let's just write it happy.
Split into simpler sentences: Initially they were writing a sad ballad about a girl whom he had traveled around the southern United States for, but who also broke his heart. However, Harmon decided to change it to an upbeat song, and said: Let's just write it happy.