In trouble

I just found out my best friend is moving away. He is the only person I've trusted enough to tell about my depression. He's the only person who really understands me. I don't know what I'll do without being able to talk to him. I don't know how I'll manage

Luckily it is easier to stay in touch with people, no matter the distance, today than it was 20 years ago when I was a teen. Today you have cell phones that don't charge you to call long distance. You have social media such as Facebook, Twitter, Snap chat, & others. True it's not the same as being in person but it is a lot better than "I guess we'll never talk again."
Hang in there. You can get through this.

I can relate to your fear of losing people who know you well or understand. When I lost my friend of many years, his actions effectively removed most of the people who mattered to me from my life.

I am sorry to hear that. The good thing is you do not have to depend on snail mail anymore. A text message is super fast. Honestly my bestest friend does not live near me but is very supportive from a distance and it still works. It will all be ok.

My best friend lives in Michigan and I'm in New York and we have supported each other long distance for years....like though a lot of difficult stuff. It isn't always easy, but it is doable.