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I Google'd your subject question "diabetic rage and depression" and received 261,000 hits. http://tinyurl.com/5llkre
"Is not willing to get help for his condition."
He needs to see a psychologist and/or a phsychological therapist for his depression and anger. Your GP can possibly recommend one.
Speaking of GP, what do you mean by, "I've tried to speak to the GP -- but because of issues of patient confidentiality, haven't made much headway." You're his spouse and by law have as much right to hear, see or obtain his medical records as he does. Write a certified letter to the GP requesting copies of your husbands medical record to be sent to your home address. Make a copy of the letter for your records too. There are places and agencies that can deal with stubborn doctors.
Good luck.