Why Is he always on my mind?

There is this guy who has pretty much always been in my life. We met my sophomore year of high school because I was dating his best friend. A few years later, I had a class with him. This was long after I broke up with his friend. We had a very strong attraction to each other, but I was still a virgin and very naive. He texted me a few times over the summer to hang out, but it was usually just to have sex I think. He would call me in the middle of the night, then when I didn’t answer he would frantically text me asking why I didn’t pick up. He talked to a mutual friend about me and the friend told me that this guy was clearly into me. But I was very shy so I didn’t do anything about it.
Anyway, one night we kissed and it wasn’t great. Like I said, I was still a virgin and a bit sexually awkward and I think that’s why. He texted me again a week later at 3am to “hang out.” Of course I declined because I wasn’t ready to lose my virginity. After that, I stopped hearing from him.
He got a girlfriend a month or two later and this is when I instantly regretted not talking to him about my feelings. I don’t know if he ever would have dated me then because we were very different but there was something there.
Anyway, I saw him a few times two years ago after we both graduated and we kissed once but since then I haven’t seen him.
I still think about him all the time. On a daily basis. I don’t know why but I can’t get him off my mind. It’s been four years since we had those “feelings.” No matter who I date, I still think about him. Every time I’m in his presence It feels like I’m burning, that’s how intense my feelings are. I don’t know what to do about it.