our family dog has died

I sit here, without really knowing what to say. our dog, Cujo, of 12 years passed away this morning. he was our first "baby", being older than our first born by 3 years. he passed away alone, outside. my heart is exploding with pain.
I knew something was wrong. I felt it. I even thought this could be happening and I did nothing.
and it wasn't from lack of caring. I loved him more than words could say. I just didn't want to admit that something might be wrong. to me, after all, he was only 12. my first dog lived well beyond twelve, and lived a much harder life. it doesn't matter. the pain I feel for him is immense. the loneliness he must've felt in his last moments. we weren't there. we weren't holding him, petting him, telling him it was okay to let go. how could I have done this to my dog? how could I have let this happen?
why didn't I go outside? why didn't I check on him? my guilt is enormous. my sadness overwhelms me.
what kills me even more is the relationship that Arnie had with Cujo. they were so close. cujo would always sit on Arnie's lap, no matter how big he grew to be, his place was there, on Arnie's lap. Arnie had to break the news to me. Arnie packed him up and took him to bury him. Arnie is doing this by himself, while I watch the kids. I don't know what to say to him. while I stood outside and cried, he merely watched. everyone grieves in their own way. it just kills me.
we haven't told Lily yet. Amelia is only 2, and Cain is nearly one. neither one will grasp what has happened.
I wanted to look at pictures of him, so I was flipping through photo albums, when Amelia lit up at the sight of him, "Cujo!!" she proclaimed loudly, squealing at every snapshot of him. my heart was crumbling as I tried to smile and affirm her shouts.
there is so much regret that comes with death. we didn't spend enough time with him. we could've made things comfier for him. we could've took him on more walks. we could've played with him more. we could've just hugged him one more time.
there's always witty sayings on facebook about living for today, about cherishing everything, because nothing is promised tomorrow. death is a stark reminder of how true those saying are. 

I want more than anything to go hug him one more time.
don't wait for death to sneak into your lives. the heartache, I can assure you, is a hard look in the mirror of what you thought was important.