Breaking up Was the Plan, the Duke Falling for the Villainess Was Not!

I was reincarnated into the pages of my favorite romance novel. Not as the bright, beloved heroine, but as the cruel, grasping villainess, a mere stepping stone in the story. My only job is to dump the "Cold-Hearted Duke" so he can meet his true love. Fail, and war breaks out. I die. Nobody gets their happily ever after. Easy, right? Except playing a vindictive noblewoman when I'd been an ordinary woman in my past life is harder than it looks. Because if that's the case, why—at the end of it all—is the duke holding me close, murmuring into my ear, "Grace... I will never, ever let you go"?!