Perhaps you're talking about the novel or the remake/adapted version of this manhua? fl didn't actually die of illness though. Her death was misinformation that they spread so that the ml would give up on trying to find her.
The adapted manhua can be found here: http://www.mangago.zone/read-manga/a_lonesome_fragrance_waiting_to_be_appreciated/
The novel translations can be found here: https://moonbunnycafe.com/a-lonesome-fragrance-waiting-to-be-appreciated/
https://www.novelupdates.com/series/gu-fang-bu-zi-shang/
I read the novel and watched the drama and you might be confused. After Pingting does back with her master, Beijie isn't told she died. Because of the treaty Beijie made to Hexia, he can't attack Yan so he goes after the country of Liang. Meanwhile, Pingting ends up leaving her master because she's in love with Beijie and her master can no longer trust her. She ends up in the Liang after her bestfriend Yan Feng asks her for help with the war. Pingting joins the war as Liang's war tactician and she meets Beijie again and they get together once more.
There is a part later in the story where Beijie is told Pingting has died but it wasn't due to disease.
I think the story you're thinking of is the novel Bu Bu Jing xi or one of its drama adaptations: Scarlet Heart (chinese drama) or Moon Lovers: Scarlet Heart Ryeo where at the end the girl gets sick and she sends a letter to the emperor to come visit her one last time. Due to a misunderstanding, he doesn't read the letter and she ends up dying waiting for him.
Guys!!!! I read this many years ago and I know for sure...100%...that this manga was a little further. Like, I remember that the fl went back with her master and the ml waited these years but then he heard news, that the fl died because of her illness.
I'm going crazy because I don't know where I read it!!! I need help here ╥﹏╥