Hey man it's all just psychological. Every time you think of your crush you feel good right? Well that's because of the dopamine that is shot into your brain every time you think about her. And dopamine is a really addictive substance, which is why you find yourself always thinking about her because every time you think about her your brain gets a dopamine rush, and it makes you feel good.
However, this is very dangerous because unlike actual drugs there is no physical action that takes place, so you are unware of when you do think of her. So if you want to stop thinking about her, you need to start being aware of when you think about her, and then clear your mind. And then every time you catch yourself thinking about her, tell yourself to stop.
There is nothing special about her. She is just an ordinary person. It's all just psychological.
If you want to learn more, here's a Ted Talk on why moving on is so hard for some people, where he gets into the specifics. It's acc really interesting and I encourage you to check it out.
Good luck!