Why is this guy still on my mind?

Do you think this feeling is love, attachment, or my ego that doesn’t want to let go?
Here’s my situation. I really like this guy for about 5 years. When I finally told him 3 years ago, unfortunately he didn’t share the same feelings as me and told me it was better if I didn’t get into anything. (A month after he had a girlfriend so I added everything up and realize he was talking to her for some time since they got in a relationship and I was the one he’d text once a week or so, replied late to my text, he only wanted to hang late nights (I guess it was just for one thing which I didn’t gave him) but anyways he said not to get in anything, I didn’t asked him why I just accepted it and tried to move on. I thought I did. I had started to have have some strong feelings for someone else a year after but I didn’t speak them and when he did, I didn’t say anything so he moved on.

Coming back to my story, I had started liking somebody else and I had stop thinking about him but this winter I had seen him on social network and decided to add him and he added me back and he ask what was new in my life (a quick conversation) but this is where it started. We don’t talk anymore(I deleted him) but ever since he’s always on my mind I’m always asking myself questions like why her, I’m always thinking about them two and it’s driving me crazy.
How can I really move on and not feel anything about him. I don’t even know what I feel but I would like to get them out of my head.
Anyone has something to say about this? (I jump a lot of details to make this short but if you have any questions or need clarifications let me know). Thank you!