Xu Mingyue is transported into a novel called The World Under Heaven's Rule, where she becomes the early-deceased fiancee of the antagonist, Xie Yuan.
The male lead in the novel is an illegitimate son of the Chancellor's household. Armed with all kinds of cheats, he eventually helps the Third Prince ascend the throne and becomes a powerful minister second only to the emperor. The antagonist disguises herself as a man and works her way up through the imperial examinations, rising from a farmer's child to Grand Secretary. She builds irrigation projects and serves the country and its people, but even her extraordinary abilities can't overcome the male lead's protagonist aura. In the end, she is executed.
The novel says little about the antagonist's youth: her family was poor when she was young, and as an adult, the male lead discovers that she's a woman disguised as a man and uses this secret to keep her under his thumb.
Xu Mingyue: This is obviously my beautiful, strong, tragic protagonist!
Now that she's here, she can keep the beauty and strength and ditch all the tragedy.
The protagonist's achievements? She takes them and gives them to Xie Yuan. The protagonist's cheats? She intercepts them halfway and makes them her own.
Xie Yuan: I don't know what's going on. Achievements are just falling into my lap.
Mindless feel-good fiction + gorgeous female lead + cross-dressing without getting exposed + two female leads
(A story written in a fit of madness. Please go easy on me.)