Try to diversify your interests and engage in more real-world activities. That way, you won't be so fixated on the fictional character. Also, think about the flaws of the character to break the idealized image in your mind.
Well, one way could be to set limits on how much time you spend thinking about them. Another is to consciously focus on your own goals and the real people around you. Remember, fictional characters can be fun but shouldn't take over your thoughts too much.
Shigaraki is not dead in the manga. The storyline has many twists and turns, and his character plays a significant role, so his survival adds to the ongoing drama and suspense.
One way could be to remind yourself that they're not real and engage in hobbies or socializing in the real world. That might reduce the simping. Also, limit your exposure to the fictional content related to those characters.
It could be that the character's story and development have really resonated with you on an emotional level. Maybe they represent something you aspire to or find comforting. Also, the way they are depicted, their looks, or their personality might have captured your interest strongly.
A 'shigaraki x femkokkuri fanfic' might explore the relationship between the two characters. It could involve character development, their interactions in different scenarios. For example, it could show how they support each other during difficult times or how their personalities clash and then gradually find common ground. It might also introduce new plot elements based on the unique characteristics of Shigaraki and femkokkuri in the fanfic's universe.