Ah yes, I used to feel this way for a bit. This is what makes life funny. The normal life span is around 75, right? 75 years isn't really that long if you think about it. Time won't slow down for anyone. You still have the rest of your life ahead of you like most of us. You can't just lay around being depressed, because that's just ruining everything. You have to live each moment of your life to the fullest.
So what if you'll lose some friends? You'll make new ones, and have more experiences, and you'll probably keep you best friends forever. If you happen to never talk to your best friends again, then they weren't exactly your best friends. You have memories to remember the good times in your life along with everything else. If you're just depressed all the time, you might as well live in a hole the rest of your life.
The purpose of your life is to make it into what you want it to be. There are so many things in this world of ours that you can do. So many fulfilling experiences, and you're sad because you're not gonna see some people ever again? Psh, if you care that much, then make sure you don't lose contact with them.
You can't live in the past. You have to live to, and for the future.