It's a historical isekai manga( not 100% sure if it's a japanese one or korean one) . The fl realises that she's a villainess and wants to avoid her bad ending where ,out of jealously she gets brainwashed by a demon and tries to harm the main couple. The ogML falls for her but she doubts his feelings. The ogFL is a saint who gets transported into their world( that's why I'm inclined to believe this is a japanese manga) and the ml feels compelled to be with ogfl but is able to resist it. Then I remember that the fl's knight is the one who gets brainwashed by the demon since he has unrequited feelings for the fl and since the demon is not able to get to the fl ,it chooses this guy. Then there's also a plot line where the demon inhabits a dog and encounters the ogFL ,she gets attached to him. They then discover that the demon/ demon's host is the ml's father's uncle and they try looking into him and the ogfl is especially interested coz she kinda is attached to him. Also I believe it has smut.
Ive always read about obsessive puppy tops but have never across any of their bottoms being equally or more obsessed than them.Bonus points if the bottom has dark hair( i have a soft spot for darker haired possessive men)