How do I get over someone I never met?

Back in 2019 I was in Barnes and Noble and this girl smiled at me, that moment made my day. She looked like she was my age, I don't know how old she was because I didn't even talk to her. I have imagined scenarios where I end up with her but I'm trying to get it through my head it more than likely won't happen. I want that to be a happy memory but not build my life around her. What do I do?