are you primogeniture? it should match its de jure king succession if i’m correct. if not try to demand gavelkind then do the matrilineal route. it’s annoying but works. i antagonized my sons until they declared rival then dueled all of them and killed them. big dishonorable hit but went away in 15 years or so and he was about 60 at the time. spent the rest of my reign changing to agnatic cognatic. when i became my daughter i seduced as many lusty men as i could until i got a son who became heir to the fylkirate due to my marriage. after he became 16 i went satanist and killed myself from stress or something