What kind of dog do you have and how old is she? Some dogs are just more drivier than others when it comes to cats and other small animals. I used to own a Pitbull/lab mix who loved his own cats but anything else outside his "pack" he would want to kill it. He killed a raccoon once in the woods when my mom had him out on a walk *sigh*. And he hated other cats. I have always had cats and dogs together and never had a problem. My ACD is very drivey about working and fetching his toys but I'm glad he isn't crazy when it comes to squirrels and other small animals especially "cats". Maybe you should get a cat from your local shelter and they can be friends LOL! No, maybe thats a bad idea....
My ACD and cat love eachother, heres one where my cat Kona is trying to get away from him behind bars!