I had a similar issue when I was younger, ManicPixie. It took me FIVE YEARS to get over my obsessive thoughts about my first boyfriend.
The key, for me, was recognizing all the other issues that were rolled into my first boyfriend. He had dumped me with no explanation on the very same day that I dropped out of college. We had dated for only six months. I had not been ready for sex when I was with him. I also had depression and was struggling to make friends. He said some bizarre things to me when he dumped me, then refused to talk to me about anything, ever again.
All these things combined to make it VERY DIFFICULT for me to sort out my feelings for him. I spent years imagining that he was the lost love of my life. I had lost my one chance at love at the same time that I lost my one chance to graduate from college.
In addition, I had a lot of kinky fantasies that I was uncomfortable with, but that I applied to him in my obsessive imaginings after he dumped me. It took me 5 years, another relationship, and then a concerted effort at casual dating / casual sexual exploration for me to sort that out.
The key was to identify and isolate each separate issue and deal with it on its own. I got a job and later went back to college. I made new friends. I dealt with my depression. Finally I figured out what I wanted sexually. Etc.
Eventually each separate issue was resolved. And by then I realized I was a completely different person from the awkward 19-year-old I had been when I met my first boyfriend. By the time I was 24 (and enjoying hot sex with a casual partner), I realized my ex would be WAY too immature for me now.
This year, 12 years after I dated him, my ex sent me a Facebook friend request. I ignored it.
Single, straight, female, solo, non-monogamous.