I saw it in cinema. SPOILLLLEERRRR ALLLERRRTTT
The girl was schizophrenic. The entire time she was home from the hospital she was having audio and visual hallucinations - I don't know how more elaborate you want the explanation to be - but hopefully someone who's seen it more recently than I have can explain more!
I just remember not liking it because I always hope for happy endings and it was definitely not one
"..I think you need to stop following misery's lead.
Shine away. Shine away. Shine away.
Isn't it time you got over how fragile you are?
We're all waiting on your supernova, 'cause that's who you are.
And you've only begun to shine.."