Damn...

I didn’t really understand what happened... care to explain?

His diary goes backwards in days, from being annoyed by a ghost on 5th day to wanting to commit suicide on the 1st, and then they show why the ghost is haunting and annoying him. The girl died because she was looking at the desolate guy in the balcony and wanted to cheer him up... so that's what she did after she was hit by the truck..