The quote 変わってしまった 変えられなかった The translation says "I've changed, I can no longer change back" But where does that "change back" come from? The verb is just "to change yourself". Also, what verbal form is this? Is it potential or passive?
The opening song describes the basic plot and the setting of the anime Tokyo Ghoul.
This answer might contain minor Tokyo Ghouls spoilers so keep that in mind.
Kaneki Ken, the main character, is turned into a human-eating ghoul against his will. He then struggles to live his life as a human being in spite of what has happened him, but he can't fight against his nature. This means for him, that he can't turn back.
The line you quoted is a direct reference to this.
I have turned (into a ghoul), I can't turn back (into a human)
By the way you shouldn't always rely on animelyrics as the source for translations of anime songs. This translation seems more accurate to me:
I quickly googled and found the original song, and also found that it was a song for an anime called 東京喰種.
Judging from the rest of lyrics, I thought "I've changed. I could not change you." is the correct translation.
However, the content of the anime should have more context. If the translation is correct, 'I could not change "the fate" or "the world"' might be the implication. Is there any fate that makes him impossible to change back?
I still think that the translation, "I can't turn back" is a little bit strange. The reason is that, to explain "to change oneself", I think we prefer to use "変われる" than "変えられる." Well, probably, "I was not able to be changed by myself." is a grammatically correct sentence but is a little bit strange. And, a similar thing happens in Japanese.
Anyway, this is really ambiguous, and of course the lyrics is intentionally made in that way.
First let's get the dictionary form of these two verbs
変わってしまった = 変わる = intransitive verb = action that has no object = ex. the flowers bloomed!
変えられなかった = 変える = transitive verb = action that has an object = ex. I picked some flowers
Therefore, "変わってしまった, 変えられなかった" means He just change out of his control (automatically change, beyond his control), then he can't change it back anymore (using his ability he can't change himself back)