Read fanfictions. There are countless amazing Harry Potter fanfictions out there that can inspire you to create your own magical world within the existing universe. You can get ideas about different character relationships, new plotlines, and unique magical concepts. Another way is to start learning the lore of the wizarding world in more depth. Know the history of Hogwarts, the origin of different spells, and the backstories of the magical families. This knowledge will help you feel more like a part of the world and enable you to create a more immersive 'fanfic' life for yourself.
Begin by creating a Hogwarts - like environment at home. Set up a study area that looks like a wizard's study, with old - looking books, a quill and ink, and some potion - making - like equipment (even if it's just for show). Then, pick a Hogwarts house for yourself and try to embody the traits of that house. For instance, if you choose Gryffindor, be more courageous in your daily life decisions. Next, start collecting Harry Potter merchandise such as action figures, keychains, and bookmarks. Display them proudly in your room to constantly remind you of the magical world you're trying to live in.