How do you stop missing someone?

I feel like everyone in my life is leaving and I can only watch it happen. I'm falling apart. 7 years ago I had just moved across the country, leaving my family and memories behind. Before I started high school, I was a loner. I was shy. I started high school and joined cross country to change things. I met my best friend. I flourished in high school and made friends and played sports. Things were finaly good. Now, I'm a junior and my best friend and most of my friends are graduating. Others, in other grades, are moving away. My sister's moving across the country. And the guy of my dreams is going to college in TN. I didn't even get to tell him how I feel, we drifted apart this past year, and I regret missing out on not being better friends with him. Now, I feel like my life is going back to how it was before. I'm alone and left behind. I miss everyone. I feel really depressed and don't know what to do. I overeat and can't stop, I cry, and I feel like I made a mistake with that guy. what should I do? How can I move on?