How do I (M18) move on when I was in love?

I (M18) cared about this girl (M21) but we stopped talking and going out and everything and now I find it really hard to move on. How do you move on from someone who you once thought would be “your person?” I’ve been asking myself questions about myself and how I’m living my life after I let this girl go and my answers keep pointing in the direction that I still harbor feelings for her, and I don’t think that will ever go away. But I want to be able to exist without always having her in the back of my head.

TL;DR How do you move on when you were in love?