Fellow dudes, how do you stop thinking about a girl or girls?

Just everyday thinking about her and other girls. I want to focus on other things like school, my job, my health. But I'm always focused on meeting girls or thinking about them. I mean I go to the gym, I focus on school, but when in spare time all I do is think about the girls.
Fellow dudes, how do you stop thinking about a girl or girls?
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