"What am I doing here. I've become a pawn, a marionette that lives for the whims of its master."
Indeed, it's not Koyomi Araragi who has control of this Tale, this Possession Tale, this Tale of the Possessed, this Possession Tale that doesn't allow for anyone not to be possessed. Not Ononoko, not Kagenui, not Shinobu, nor anyone else - they're no longer the protagonists of their own story.
At least, so it seems.
After all, what other explanation could there be behind this farce of a tale? Where nothing is as it seems, and yet, everything is exactly as you'd expect. Where they conclusion is all so obvious, but our oblivious narrator fails to grasp what he should have realized ten books ago.
The candle isn't at its last straws, it's already burnt out, this story has already come crashing down, and the fire of the long-gone candle has catched on the house, burning it all down.
It's all falling apart.
The narrative itself mocks our protagonist. "What are you even DOING, the Monster Tale is nothing more than an Ordinary Tale now; it's all come clear by now, so why, just why, why, why, why are you still refusing to accept it?"
"Just why?"
"Just why are you so fucked in the head?"
Koyomi Araragi is the one who hates himself. He who sacrifices himself for others. He, the total hypocrite, who started this Monster Tale by choosing to end his life for the sake of some vampire, in modern Japan of all places - just how ridiculous is that?
It's as if we...as if us humans haven't grown past stage yet.
And yet.
No matter how ordinary all these monsters, whether vampire, ghost, crab, snail, monkey, doll, you name it, no matter how ordinary it has all become, we are still at the starting point.
Even though it should have been solved centuries and centuries ago, even though Araragi should have know for ages and ages that he should cherish himself, that he should live for himself, not others, he can't help it. He can't help getting caught up, and wasting away, in his trauma. No matter what, no matter how much he's learned, no matter how much society has supposedly grown past Monster Tales - no matter how ridiculous an encounter with a Vampire may seem - he just CAN'T learn. Can't move on.
Possession Tale has a dominant theme, which Koyomi verbalizes only once, in a seemingly random one-off note, almost like a footnote, like a random quip that has nothing to do with the story at hand. And yet, it is paramount to understanding the full picture: The dichotomy of knowing that you don't know something, or not knowing that you don't even know something.
Not knowing that you don't know something - I mean, that's completely fine. In fact, it is the entire origin of humanity's strive for knowledge: They know that they don't know about something, and so they search the universe for an answer. They strive. They aren't perfect, but they nevertheless fight to become better, and that's exactly what makes humans so beautiful. They are Fake. Nisemonogatari. That's the entire point of Nisemonogatari.
However, this Possession Tale, is a tale of the latter: Those who don't know that they don't know. Even though they should. Those who've grown stale, never chasing for the answer for something, remaining oblivious of their own obliviousness. Those who refuse to change. Exactly the kind that Araragi Koyomi is. Even though he should know for a long time now - that the one thing still to clean up, that what these Monster Tales have been preaching from the very first Tale, the very first pages, is to get rid of his self-hatred. His self-sacrificing, altruistic nature. However, he refuses to acknowledge it, like the fool that he is. He'd rather bumble around aimlessly, not knowing what's even going on - "Oh hey, Ogi, oh man what a quirky underclassman, don't you stay up too late, you hear me?" - than change this simple yet fundamental flaw of his character.

And so, we arrive at this Possession Tale. Where everything is seemingly possessed, already orchestrated fully, before it could even begin. Because, really, there's not another outcome to this Tale. What else could there be to talk about, after all Koyomi has talked about Monsters for so long they've become the Ordinary - even though they never should have become ordinary in the first place. Trapping himself in this maze of monsters, he stopped recognizing just how monstrous, just how inhuman - just how No Longer Human - he has become. And that the solution was right in front of his eyes the entire time, a a forest hiding in all the trees.
Because the solution is not to give up on Monsters, nor is it to become one. The solution is neither to be perfect, and therefore inhuman, nor is it to become a monster, and therefore inhuman - the solution is to mediate between the two sides of the same coin, and therefore FIGHT. Fight to be a human. Fight his flaws, even if he may never be perfect. To be fake, instead of being either of the real deals. To realize his flaws, instead of pretenting they aren't there, instead of ignoring them in favour of yet another foolish, self-serving tale of self-sacrifice. So that the possessed may cease, and open the room for the next Tale, one that's neither possessed by monsters nor by perfect things - but one about finally finding true happiness.

After all, Koyomi - you silly little thing.
Don't forget -
To cherish yourself above all else.