Lf mangga shojo

Fml is transmigrated to a novel where she is the villainess (cliche). Broke her engagement with the yellow haired prince, and fall inloved with the demon king. Its a fast pace story. And then the fml of the novel arrived which is also from the outside world and knows the story. Took advantage of her knowledge and slack off. (Fml of the novel is like the hero or saintess they waited for). But the female lead (villain) did all the route that the original hero should do in the novel and had a happy ending.
I cant really remember the details