As much as I'd like to hope it's not option two, that seems to be the most fitting reason for all the nightmares and insomnia etc...
My poor baby (个_个)
Yah I could say it's the "best" option from what I learnt in my classes. I'm just a freshman so I might be wrong tho! But all the flashbacks with this certain person it gives me goosebumps :(((
Our poor baby TTATT
In chapter 74 we can see that he got gang raped I wanna die
I think there's two options about seungho's past :
-the "disease" could be him being gay. I think he told his father about his feeling or whatever and this homophobic mf took him to see a doctor coz he's "mentally ill". Which could explain why the doc said he wasn't ill. Maybe Seungho went mad coz of all the pressure and stuff and now that he's far from his father he lives a life of debauchery with mens to sort of "provoke" him coz he knows that the servant or whatever tf is tell all this to his father.
OR
-I really hope that it isn't it God pls. Maybe someone sexually assaulted him or raped him when he was younger. I'm in psychology faculty and we studied some post traumatic desorders and usually the victim suffer from insomnia, hallucinations, panic attacks... Sometimes a victime of rape or sexually abuses at a young age forget all this for a certain time and suddenly remember when they are being a adult or with a certain experience (sex with the partner often). Maybe he remembered when he turned 20 or something and that coulb be why the doctor saw him "the second time" in a euphoric state.
I think this 2 options would be closer to the plot in my opinion, btw sorry if I made any mistakes don't hesitate to tell me thanksss :))