HELP me find a book plz. Emenies to lovers

[Hugo]

There's this book that I have read a couple years back so my memory is a bit fuzzy but here's what I remember.
World building is historical. FL is a princess. She is a naive young princess, live in a small but happy kingdom. One day she got engaged with the bigger nation's crown prince (ML). Her family help her to fled away from the palace whilst they die. She then disguised as a man, choose new name, join the ML army and then gradually gain power. After a few years, she got close to ML, become his direct subordinate. After some drama and war stuff and stuff, they develop feelings. ML know or her identity but choose to keep silence, FL struggle to accept her feeling. In the end, FL kill ML ( partly ML suicide, sacrifice himself kinda to help her avenged her family). There's also a side couple. Side couple in the end ran away from the politics and stuff and live a happy life. FL suicide also. From what I remember, It's a 2 parts series. FL is a capable, cold, caculative and intelligent. The story also involved some pretty good war tactics and stuff. ML is badass, FL is badass, those two are black flags couple made in hell. BUT I LOVE THEM SO MUCH. I forgot about them a while ago and I'm bawling my eyes to find and reread them again. PLEASE HELP.