Iíd like to write this down to better explain what I feel has happened to your dog.
You have raised your dog with lots of love and attention. She is a wonderful dog to be around! She is interested in working and will bite cattle. There is something missing, however. She does have the desire to herd, but lacks the "NEED" for it. Another way to put it, is: She doesnít have an Obsessive/Compulsive desire to work livestock. Most border collies have this. It can drive you crazy at times, but it is the key element in allowing you to train a working dog. Intense Desire! What I feel has happened to your dog I call the "Kid in the candy store syndrome". He has been exposed to stock so often growing up, that he doesnít see them with the same "Hunger" as a dog does who was raised without so much time with stock. Some Border Collies would still be able to be trained to an acceptable level. (Some dogs just canít be messed up) Your dog will go out to cattle and work them, but if a cat runs by or some smell catches her nose, she goes to investigate. If you scold her for it, she comes back to you and wonders what you want. "Whatís the Problem?" the dog thinks, "you always let me sniff around and chase the cats growing up". "If I chased the cows, youíd let me do it a bit, then call me, so whatís the difference now?" Can you see this from the dogís point of view?
It isnít just stock either. If you allow the dog to run around and play all the time, (with no stock around), it seems to develop a desire to "Just do what it wants to do". If you put too much obedience on a dog, that affects the dogís herding training also. There is just a FINE LINE either way. Itís hard for a first time Border Collie owner to understand. Iím trying to help you see what went wrong.