this song is about losing someone who commited suicide. if you pay attention to the melancholy, drawn-out way this song is sung, you can definitely understand the pain she went through. I almost lost a good friend to suicide, so this song makes me happy that he is still alive today.
It's the girl herself, talking about losing her true self.
Because she didn't treat herself well, she's who she really is.
There fore, she lost herself.