I can’t speak to why you feel what you feel but if it’s the feeling I’m thinking of - it’s because you tend to define yourself by what you “know” or “have experienced”. If you become familiar with a piece of media like One Piece or Star Wars it becomes a part of you, defining to who you are as a person. If nothing else, you’ve spent some portion of your life consuming that thing and you’ll never get that life back - life that you could have spent doing something else more unique and worthy of the image you build of yourself in your head.

I recommend letting this feeling go as best as you can; it’s very freeing.