I found spoilers in the novel so thank me

First timeline: Linzel got together with Mr Handsome so Likke accepted her arranged marriage with Herman but they didn't get along so Likke tried to get the sigils to the secret chamber to find the cure by herself and Marchese Miller (the villain of the story) was being a bitch about it, before entering the secret chamber Likke learned Linzel died of the incurable illness and a broken heart because Mr Handsome cheated on her. Likke got super depressed and reclused herself like the third prince. During Miller civil war, Apophis/Mercy went to Likke because Linzel's death remembered him of his wife Leonia death ( Linzel and Likke ancestor with the same incurable illness) and Likke learned that Apophis can make her regress time so she could try to save Linzel.

First timeline: Linzel got together with Mr Handsome so Likke accepted her arranged marriage with Herman but they didn't get along so Likke tried to get the sigils to the secret chamber to find the cure by herself and Marchese Miller (the villain of the story) was being a bitch about it, before entering the secret chamber Likke learned Linzel died of the incurable illness and a broken heart because Mr Handsome cheated on her. Likke got super depressed and reclused herself like the third prince. During Miller civil war, Apophis/Mercy went to Likke because Linzel's death remembered him of his wife Leonia death ( Linzel and Likke ancestor with the same incurable illness) and Likke learned that Apophis can make her regress time so she could try to save Linzel.

Second timeline: Likke talked to Linzel about sir Handsome cheating and made them break-up, being single Linzen meet prince Ishar at a banchet and they click. Likke refuse her marriage with Herman because their failed marriage in the first timeline. Herman married a woman of another family. Since Linzel and Ishar/Izeckel fell in love and got married Likke decided Linzel was in a better position to try to go to the secret chamber but Linzel illness got much worse than before. Also Miller kill her in his revenge of the royal family like he did with Illya's cat. After Likke failed to get proves of Miller's crimes (she knew he was the culprit because he hated the royal family in the first timeline) she waited for the civil war so that Apophis would come to her again (Apophis promised he would only show himself after blood was spilled to punish the foolish humans). Likke begged for another chance and Apophis confessed that saving a person destined to die was almost impossible, Apophis himself didn't managed to save his wife even if he turned back time, he found the cure of his wife illness only after he couldn't turn back in time anymore (not much time must pass between the death of the person you want to save and the regression). Also because interfering with time is against nature there would be side effects and since Likke's desire was to save Linzel then Linzel's illness would became worse and her memories altered. Likke still didn't want to give up and she regressed a second time. She also thought prince Ishar was too weak to protect Linzel and Herman would be a better fit ( even if Likke own marriage failed she had the confidence that Linzel would manage to melt Herman's cold heart). Apophis/Mercy also forbidden her to speak about the regression to Linzen because it would be more amusing for the dragon.

Third timeline: Because the side effects, Linzel mistook the plot of a book she read in the imperial library in the second timeline and joked with Likke about some similarity with their situation as her transmigrating in said novel. Likke also faked her love for Ishar/Izeckiel so that Linzel wouldn't get interested in him and choose to get in the arranged marriage with Herman. The rest is the start of the novel/webtoon. During the day Miller start his revenge plan, Likke changed her appearance to look like Linzel so that Miller would be confused and shot her instead with a gun. In that moment Linzen started to remember the memories of the past timelines and got confirmed by Mimi that brought Likke's diary to Linzel and Mercy testimony. Mercy was amused that since he has forbidden Likke to talk about the regression she wrote it instead in case something happened to her LoL. While Likke was in critical condition, thanks to Mercy and Likke's diary, Linzel found the cure of their illness but too late and Likke died. So this time Linzel begged Mercy to turn time back to save Likke and strike a deal with him so she regress together with Herman. Returned to four days before Miller attack, they managed to get the cure, defeat Miller without casualties and Likke and Linzel finally can be honest with each other and live together happily.