Why can't I move on from a rejection that happened a while ago?

So I was rejected by this guy almost 2 years ago and I still have a hard time letting it go. It's not like we're best friends. We met less than two months before he'd reject me. I haven't seen him for more than 5 months and haven't texted him for 7 months. No social media stalking. I went on a date with several guys and tried to make myself busy with a lot of things and I still can't stop thinking about him. I'm exhausted by this at this point and I want to save myself from the pain. Please lmk if you have been in a similar situation.
Why can't I move on from a rejection that happened a while ago?
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