I feel guilty

We had to leave our labrador retriever in Michigan, a couple of our neighbors were taking care of him, this elderly lady just loves him, he came up missing last winter for a couple of days we found him at the ladys she took him in and made a bed in her garage for him gave him his own food bowl. The neighbors called yesterday and told us he wasnt doing good at all, I was going to call my sister today to go and take him to the vets, but this morning she called and said he died.
Every morning he would walk with me to take the kids to the bus and on the way up I would throw a ball, and he would bring it to me. I cant even count how many basketballs he'd find and bring home.
I should have been there.........he died alone on the neighbors porch, I dont even like to think about the fact that I am sure he felt abandoned by us, but I cant stop thinking about it and then I start crying, I should have been there.
We have decided to not tell kids yet, were not there and dont plan on going back to visit till Christmas, we will tell them then. Hubby is flying back in a couple of weeks for deer hunting, maybe when he comes back from that we can tell them then.
Shannon one of the neighbors that was watching Duke for us said the elderly lady is taking it hard, We had hubbys brother and dad go over to take care of him but when they got there the elderly couple already dug a hole for him in their back yard and buried him.
Tommorow I am going to call my sister and have her take the elderly lady some flowers. Its the least I can do. I feel bad that the burden had to fall upon them but glad that he was there, he knew he was loved there. I will and do miss him, we were going to bring him down if we stay here, but had no room in this apartment for him. I do feel guilty for not being there and it will weigh on me for a long time, thinking he most likely felt abandoned by us. signing off for now