Okay first up:
1) don’t catfish her like someone suggested. Tricking her into thinking you’re someone else isn’t going to make her trust you or like you. 0/10 worst advice ever.
2) don’t keep trying with this girl, she’s made clear that she’s not interested and feels uncomfortable. Keeping trying only makes her uncomfortable and you more wrapped up in this fantasy.
I know how you feel, I’ve had feelings with a version of the person that I had in my head. Getting over the imaginary them that I loved was much more difficult than not seeing the actual person anymore. All of the “what ifs” and things you’ve lived in your mind are hard to get over, since you never got the closure of doing them. I can assure you that you aren’t the first person to feel this, and you will get past it. It’s just going to take some time.
My tip is every time she pops into your head, think about something else. It sounds silly but “changing the subject” as soon as she steps in is how you get through this. When I find myself starting to think of that guy, I actively think of something else. Anything at all that will distract you. You like Michael Jackson right? Imagine you’re at a concert, or that you have the chance to meet him.
Physically distracting yourself is good too, if you can’t get her out of your head start doing something that requires a lot of focus and mental energy. Something difficult or something you enjoy.
The trick is to realise that it will take time for this feeling to go away, and dwelling in it isn’t going to speed it up. In a few years you’ll look back and wonder why you were so worried, so wrapped up in her. You just have to accept that it’s done, and let those parts of her in your mind move from the front to the back, and then out the mind completely.