How to get over a guy

So I’m in 10th grade, and I’ve always had crushes on people. Like major ones that define me. Right now school just started and there’s this guy in my class that I’m crushing on hard. Obviously my period hormones right now are making it worse, but even when I’m not on my period I’m distracted and thinking about him. It’s a common trend in my life. How do I not let my brain get dominated by one guy or girl?