You are missing him so much because he was your only friend.
That means you need more friends right away. Building up a friendship is not done in one day, but it's a good and healthy target to work on.

You feel like you've lost everything, but that is only because you did not have enough, enough friends, enough people to go around with, talk to and have fun.
I believe this is the first and most important goal you should set for yourself right now.

Keeping busy is a very good advice to shift your mind from thinking about him only and all that you lost.
Focussing on making more friends is a lifetime goal so that you won't feel as lost and lonely as you feel now. But of course you feel this way if he's the only friend you had.

Focus on YOUR life, not on him. Let him go. Forbid yourself to think about him and get busy with your new life.