Can someone help me find this Manhwa: I'm desperate

Genres that I remember from the Manhwa:
Romance
Isekai
Historical

The main character (The new female lead)

• Mc is isekaied into a novel.
• When she wakes up after isekaied she is in bed with a man(the male lead / her new husband) before the mc had been isekaied The og and ml had just spent their wedding night together.
• The mc gets out of bed and walks over to a mirror and sees her self the male lead wakes up walks over and finds letters on the mirror stand addressed to and from the person that the female lead had just been isekaied into and to and from multiple male suitors.
• The fl later burns all these letters.

The person that the female lead possesses ([quote])

• Black hair(?)
• A villainess.
  -She had multiple affairs
• She was an amazing horse rider and won many awards for it. While the female lead can't ride a horse.
  -Got killed in the novel by a rival king along with her husband.

The Female lead in the novel

• The male lead and her were childhood friends.
• She fell in love with the king from a rival kingdom and then the king betrayed her.

The scene that I remember is that the female lead receives an anonymous letter stating that the mc needs to meet them in a set number of days in the woods, the mc doesn't know who this letter is from as it was most likely someone that the og knew. Anyway she goes to the place and something is thrown at her from the bushes, then she completes a bunch of tasks and finally the person who was throwing the stuff from the bushes reveals himself.
It could be a manga but I'm pretty sure it's a manhwa