Hi Toby J. I am so sorry for your loss. I also lost my best friend to cancer, she was my younger sister. We shared a life history, knew each other better than any one else could, supported each other through the bad times, laughed and enjoyed the good times. She was my soul mate and every day person.
In many ways you did lose a "Brother." Long term, close & loving friends become our family.
I relate to your feelings, of wanting to retreat from the world, because it is just too painful. I have a demanding job, and like you, I lost focus. I agree that nothing else matters now. Your loss is more recent than mine, but even a year on I still cannot accept that I will never see or speak to her again. Other friends are important, but they will never fill the void left by that one special person who brightened our world.
Try not to make any major changes in your life while you are in such a deep state of grief. I also wanted to pick up and move away, but our feelings go with us no matter how far we run. Post on this site, and vent whenever you need to. Hold your friend/brother close to your heart and let him guide you, I call out to my sister every day. Take care, Sister2