Ask HN: How can I let go of a relationship that ended long before it even began?

We've known each other for a long time; he was my elementary school classmate. Later, we went our separate ways, but in high school, we became classmates again. He loved me a lot at that time, but I didn't feel the same way, so we never got together. As we grew older, I got married, but I often dream about him and feel regret that we never ended up together.