It might be hard because you're really into the characters and the relationship portrayed in the fanfiction. The stories can be very engaging and make you feel a strong emotional connection. Also, if you've been consuming a lot of it, it's like a habit that's hard to break.
One way is to focus on other interests. For example, pick up a new hobby like painting or reading non - fanfiction books. Dive into different fictional worlds that have nothing to do with Taekook fanfiction. Another option is to limit your exposure to content related to it. Stop following accounts or groups that promote such fanfiction.
I'm not entirely sure if there is a book titled 'Out of My Mind' that is specifically historical fiction or nonfiction. It could potentially be either, depending on its content. If it's a story set in a historical context and the plot and characters are fictional but the backdrop is historical, it would be historical fiction. If it's a true account of someone's experiences or a historical event, it would be nonfiction.