So very sorry to hear your news Gypsy. I know how you feel, I had a GSD with severe HD (the vet said she should not even be walking!). I lost her at 6 years old - she developed bone cancer in one of her hind legs. I think it was probably caused (or set off) by the HD. I was absolutely devastated, she was the first dog I had lost, I had to make the decision of no treatment as the only treatment was amputation of the leg and the other hip would never have carried her weight.
There is nothing I can say to make you feel better.
You did the best you possibly could. At least you shared his life for the few years he had and made him as happy as he could possibly be.
I have happy memories of my girl, running flat out on the beach, jumping and chasing with my other two girls, I know that the five years I had her she never felt lonely, frightened or hungry.
In time you will remember the good times, the happy days with him. Take the time to greave, it will get better, but for now just take each day at a time.
It cannot help you I know but my question
http://www.theanswerbank.co.uk/Animals-and-Nat ure/Question237612.html especially the answer at the end by neil refers to exactly what you are going through.
Thinking of you - take care.