I would say Angel at war fits about 98% of what you're after.
Main character is a girl that is a reincarnation of an angel. The angel used to be in love with the demon king. The girl she is now is a human who's people directly oppose this king. The king finds out she's the reincarnation of his beloved and holds her hostage and tries to win her affections in any way. Though she already has a love interest back on her home world and remains true to him for a long time, but she does waver eventually. The male love interest also murders many of her peers in the beginning, only leaving her alive. There is also a large age gap because he's a demon and she's been reincarnated already lol.

There is also Brasilis no Musume, this has a female protagonist and she's in love with the male lead. Then later in the story someone after the same thing she wants appears and falls in love with her. He uses much creepy, underhanded behaviour to make her his, including kidnap, blackmail...all sorts. Doesn't fit as well though because she doesn't love him back.

It's not a 100% match, but perhaps Nisekoi would be something you like

The Legend of Maian has some of those elements, but its not exactly a perfect fit. Also Akame ga Kiru! fits almost perfectly, but the roles of the male and female are reversed.