im so sorry. Please know you are not alone. I think most of us in here have gone thru this at least once. We open our hearts to these wonderful wee creatures and they make it bloom. I have lost several of my beloved friends, and I always knew going into a new pup that one day "that day" would come but of course you do it anyways because the love they give and leave behind is so much better than never having it at all. I lost my maltese Spike in Oct 06 and I still miss him dearly. He too was a diabetic and he died at the vets alone. It was very sudden and the guilt I felt and still feel at times was overwhelming. The could I's and the should I's haunted me. but, god knew how things needed to be and so they were. I had to learn to accept, not agree, but accept. And forgive myself for not being there even if it was 4am. Had I known, I would have lived there with him.
When your heart is ready, love will find you again. A different love, but a special one all its own. May God bless you and keep you tight in his loving arms until you find peace.