Diana(Niall Horran)

What is you were Niall Horran's girlfriend who is battling high class depression, but hasn't told him yet. It hard for her everyday to not have the strength to tell him that she has depression. One day he finds her talking to her friends about it. He wants to do something about it but he doesn't know what. Then he has an idea! Write a song for her. He names it Diana. How does she end up having the courage to talk to him about this? How does Niall react?