How do i stop thinking about her?

hi, there was a girl at my college and she and i sat in the same cl a couple times and exchanged a few sentences. I used to see her around and she always used to say hey to me. Now she doesnt goto college here anymore, BUT i am totally in love with her, i think about her everyday, and i havent seen her in months. I really dont want kids, but when i picture her, i picture her having my kids. I see us on the beach and growing old together. I never been so obsessed with any other girl, it has been a problem before where i get obsessed wit a girl and infatuate - does thsi happen to all guys? But yeh i try not to think about her but she comes to my mind everyday, i just want to stop thinkin about her coz i dunno if ill see her again or even have a chance. thanks