I lost my father due to complications of diabetes 8 years ago. I am also diabetic . . . so I know what you've experienced, but I can't claim to know how you feel. That's something private that each of us must deal with individually.
I can tell you that the hole in your heart, that place occupied by your friend, isn't really a hole. It is all pain and hurt now; in time, though, it will fill in with love and cherished memories.