How do they move on so quickly…

I still think of him a year and 6 months later. Nearly every single day. I still am hurt for what he did to me. While he’s fully married and moved on. I’m a distant memory to him. All the love he felt for me is all for her now. I’ve been single this entire time trying to focus on myself. And I just have days that are really hard. I still don’t understand how someone can say they love you so much one day and the next day you are nothing. Do I ever even cross his mind?