I'm guessing the "this is going to end soon" means she's going to cut herself off from Kou so she can forget about him (just like how she's forgotten about Kagura).
She knows that Koh's goal is to become a vampire and be with her forever and because she can no longer fulfill that requirement at the moment because she will die when she bites him (at least that's their theory).
Maybe the long winded plan is to disappear for a long while, forget about Koh, and at least reset her feelings for Koh and give Koh a second try in properly falling in love with her? Who knows, but she "knows" that as long as she's in love with him, she cant fulfill his wish.
One thing to note is that it's still unknown for a fact whether or not biting a human that a vampire loves kills the vampire. The character's in the story have only stated that as a theory, but it has never been stated as a fact in the world building. There is still an important discrepancy that needs to be addressed.
In the last panels of chapter 147 you see Kiku realize that she's fallen in love and has essentially momentarily become human when she feels a heartbeat then has the crazy face when realizing her dream is coming true (remember that is is stated that vampires do not have heartbeats stated on
page 7 of chapter 93.
Then you have
the last panels of chapter 169 where Nazuna realizes she's in love with Koh, but there is
no heart beat. So while Kiku must have suddenly "lived out" all of those years as a human the moment she turned back, the same might not apply to Nazuna because she may not have been human to begin with (remember that her birth is still a mystery)
So there is still some world building where characters will learn more about vampires and possibly find a workaround especially when Nazuna is a "special" vampire and the rules that apply to others don't apply to her. So I'd wait and see what the author cooks up.
edit: of course I might just be over thinking and she's just going to bite Koh thinking that she will die (and maybe he will turn into a Vampire) and end it that way only to realize she's fine or something alike.
another edit: Someone pointed out that I forgot that Koh can't turn into a vampire after one year so my initial theory is already bunk >.>, I'll just wait till next week for an answer.