I am deeply sorry and feel your pain. He was a beautiful dog and I am sure the very very best of friends. It will be hard for some time, I am not saying it will ever get easy but you get to another point and you learn to live with it and remember all the funny, wonderful and sweet things you had together. Also please remember you gave him a wonderful life with lots of love and good care.
I know about the guilt part, I lost a Bullmastiff to an accident that could have been prevented, I have still not posted about it here, even after a year and two months it is still making my cry every time I think about him. But dont be too hard on yourself. Eventually you will get to see that you loved your dog and would never ever do anything to harm him, accidents happen and thats how life is, tough, horrible and sad, but you never ever would have put him in a situation where he could be hurt, had you known about it. So please try and not feel guilty, I know that is hard though.
He is at the bridge now and knows he was a very loved dog with the best parents a dog could have and will forever be yours, and patiently waiting there for you.
I will spread this information to as many as I can, I work in a lot of rescue groups and have a lot of contacts, so in Dartagnons memory I will do my best to forward this.
My thoughts are with you
Marit