https://www.mangago.me/read-manga/grieving_you/
https://www.mangago.me/read-manga/when_the_ending_of_the_unplayed_game_approaches/
Not sure if they r exactly what u r searching for but I think so( ̄∇ ̄")
They're not super dark or anything but definitely obsessed and clingy:
https://www.mangago.me/read-manga/love_plan/
https://www.mangago.me/read-manga/tonari_no_banken/
https://www.mangago.me/read-manga/kimi_no_tabegoro/
https://www.mangago.me/read-manga/shitteru_yo/
Super loyal love interests who veer on the darker side? I just like crazy dudes but, at least in bl, they're always manwhores which I don't like. Like be obsessed and clingy for a reason. It has to be because that person you're pining for is worth all that.
Het or bl or even gl is fine too(gl is in fact preferred but ik they're harder to find)