I still think of another

I am happily in a relationship with my bf but my mind always goes back to him. I don't know what to do. How do I stop thinking of him

Edit-

He was just a fwb I had while me and my bf took a break. He has his own relationship. He messaged me a while back saying he still thought of me. We decided it best if we blocked each other. I don't live near him anymore. I moved recently. My relationship with my bf is long distance. I really do love my bf, I'm just still thinking of him.