Why can't I move on 1 year later?

Anonymous
So today is exactly one year since I had my first kiss and started dating this girl I really, really liked. However, like a month later she just left me and I was really hurt. She didn't tell me why and wouldn't respond when I tried to contact her, and all this time later I'm still left wondering if I did something wrong or if there was someone else, or if there was some other reason. I've thought about her every single day since then, and with this being one year to the day I just can't stop thinking about her lately. Why can't I move on? We only were together for a month, and I feel like most people would be over it by now but I just can't get her out of my head.
Updates:
+1 y
...
+1 y
Update
Why can't I move on 1 year later?
2
2
Add Opinion