Sorry to hear about your loss caligirl. I don't have human children so my pets are my babies. Just 'cause they're a different species makes no difference when it comes to losing a member of the family.
4 weeks ago we lost one of our cats. Got bitten by an Eastern Brown snake (2nd deadliest in the world). The cats have a cat enclosure & the snake came into this. I wasn't out riding that time, but I was in the computer room. I felt guilty about that. The thing is, no matter how hard we try to protect our loved ones, s..t happens. As others have said, dogs & cats etc do not react the same as us when unwell. You loved your dog & your dog would have known this. You can't be with a pet or a person 100% of the time.
Try not to be too harsh on yourself. Your dog had a way better life than many on this planet. Let that thought comfort you. Take care.