From when I listened to it I started really liking it, but when I checked the lyrics (like 4 years ago or even more) they resonated so much with me and I didn't have an idea as to why they felt so relatable. I was like "what do I mean when I say there's another one inside of me? Why do I relate so much though?"

I started putting it everywhere, from agendas to shoes to everything.
Listening to it with the knowledge I have now, it just all makes sense. It's still very calming and comforting to me.
The song is an anime opening from a sort of horror anime (Tokyo Ghoul) that many people know about.
I'll post the translation here in case somebody wants to read it. It's a bit sad but what good song isn't.
Link: https://www.youtube.com/watch?v=Cp5-0dZH55g
Tell me, please tell me, what this plot is all about.. Exactly, who resides within me?
I'm broken, so broken - amidst this world. Yet you laugh, blind to everything.
Being as broken as I am, I hold my breath,
And it can't be unraveled, it can no longer be unraveled.. Not even the truth. (Freeze)
Breakable; unbreakable - psychotic; unable to go insane
I found you, and..!
In this shaken, twisted world, I gradually become transparent, unable to be seen.
Please don't bother looking for me; don't stare at me..
I merely don't want to hurt you, inside a world, that came out of someone's imagination.
So please remember me; as vivid as I was.
Infinitely spreading, solitude wraps around me. Memories of times I laughed innocently comes to mind,
And I can't move, can't move, can't move, can't move, can't move, I cannot move!
Unravelling the world
I've changed; I couldn't go back to the way I was.
As the two twines around one another, the both of us will perish away..
Breakable; unbreakable - psychotic; unable to go insane
I can't afford to let you be defiled!
In this shaken, twisted world, I gradually become transparent, unable to be seen.
Please don't bother looking for me; don't stare at me..
In a trap of solitude someone had set, before the future unravels,
Remember me; as vivid as I was.
Please don't forget me, don't forget me, don't forget me, don't forget me!
Paralyzed by the fact that I've changed,
In a paradise filled with nothing but unrecoverable things,
Please remember me.
Tell me, please tell me.. Exactly, who resides within me?