How can I get over someone I can't have?

Back in 2019, a girl smiled at me in Barnes and Noble and I've think about her a lot. I have pictured scenarios where I end up with her but I know I can't have her. I don't know anything about this girl, she was a random stranger. I want to stop obsessing over her. I picture various relationship scenarios with girls usually my age to two years younger, I had thought this girl could be my age but I know she might not be. I want to stop thinking about her. What do I do?