How do I stop thinking about someone who ghosted me five years ago?

Posting this again because it got deleted the first time. Hopefully, it doesn't get deleted again.

I, now (23F), don’t know how to explain my relationship with (25M). We were closer than friends but never officially lovers. We met in 2015, while attending the same high school.

For the next two and a half years, We talked day and night, all summer long. He taught me about his major, the same one I’d later study. We had inside jokes, exchanged sketches and chocolate, watched movies together and had a song we’d notify each other about when it played on the radio.

Then, a few weeks before he graduated, he ghosted me. Months later, he resurfaced, opening up about how his struggles made him withdraw from everyone. I forgave him because, despite everything, he meant the world to me.

By the end of 2019, he ghosted me, again, this time after an argument. He said he’d talk to me later. Later never came and it’s been five years since.

I deleted his number, removed him from social media, and moved away, yet he still lingers in my mind. We have mutual friends, but no one questions why we don’t talk anymore. Every now and then, when I see someone who resembles him from afar, my heart would race.

I don’t want to reach out, I refuse to let him think he has any power over me. But why, after five years, is he still in the back of my mind? How do I finally let this go?