So sorry for your loss. In 2 days it will be a year that I lost my lab and it was one of the hardest days of my life. No one will ever love you the way a pet does. I can tell you it does get easier, and eventually you and your daughter will be able to talk about the good times without being too sad.
We just recently got a Newf puppy, and while she'll never take the place of my lab, there is a new place in our hearts for her.
My lab had the breathing problems too, and the vet said it's something they and Goldens get, she was on Rimadyl for pain for dysplasia, and while making that decision is one of the hardest, for them it's the best. The one thing that comforted me was that she had no idea. They really don't know what's going on. She loved the vet and walked in wagging her tail. Just know you did the right thing for your baby.