There are many series that have some sort of a female antagonist that is competing with the FL for the love of the ML. A previous love that came back from overseas and wants to get back together with the ML. Or the ML has strong lingering feelings for a woman that moved away. These series are too many to list :)
But for your description I think this series fits the best:
Goong The crown prince loves another girl and proposed to her but got rejected, the FL is the secondary choice for the marriage.