This is a restart manga which i read few years back.
The girl has a harsh life and a terrible fiancée because of this, she dies and wakes up again as her younger self. Determined to avoid the inevitable future, she starts to learn swordsmanship and becomes very good at it, sort of like a genius. Then in order to avoid the future she already experienced, she decides to join the army which a prince leads. She very quickly becomes acquainted with the top brass and fights with the prince.
The setting is medieval with dukes and counts. The girl is a noble's daughter.