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.
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.
One common theme is the 'opposites attract' concept. Given sakusa's quiet and clean - freak nature and atsumu's more energetic and sometimes messy personality, this contrast often forms the basis of their relationship in fanfictions. Another theme is their mutual growth. They help each other become better players and people in many stories.
One popular type might be the ones that focus on their rivalry turning into friendship and then something more. For example, there could be a fanfiction where they start off competing for a spot on the team but gradually realize they have a deeper connection. Another could be set in an AU (alternate universe) where they meet in a completely different context like at a music concert and their relationship blossoms from there.
There are many out there. 'Opposites Attract' is a really nice one. It delves deep into how sakusa's reserved nature and atsumu's more outgoing and flamboyant personality play off each other. The fanfiction does a great job of creating situations where they have to rely on each other, leading to a beautiful relationship development.
Some Tumblr blogs also post or recommend Haikyuu fanfictions. You can search for Haikyuu fanfic blogs and then specifically look for Sakusa x Atsumu stories on those blogs. The bloggers often give a little summary or their thoughts on the fics, which can be helpful in deciding if it's something you'd like to read.
Since it's a fanfiction, the idea of Hinata cheating could be a very unorthodox and out - of - character concept. It might be the result of the author wanting to create a controversial or alternative storyline. But in the official Naruto lore, Hinata is not associated with such behavior.
Perhaps in the fanfiction, Hinata wears a micro bikini for a special festival in the Hidden Leaf Village. Naruto is really excited to see her in it. They participate in the festival games together and Hinata's new look makes Naruto see her in a different light, more as a strong and confident woman.