I agree... your dog does not know the "leave it" command thoroughly enough to be trusted in a distracted environment. Balancing a treat on her nose and telling her to leave it is more a trick than knowing to leave it.

Until your dog can do things in a distracted environment, they do not fully understand the command you are giving them. I would back up and start at step one with the "leave it" obedience command. Once you feel she "knows it" in the house then take it outside around noisy situations and distracted environments.

As an example, when I am proofing my dogs before an obedience competition, I take them to dog stores and work attention with them while people are trying to pet them and everything else. Anyone who has gone to a dog store knows the distractions they present. If my dog doesn't pass the test there, then I know they are not ready for the ring. Use your imagination in getting your Golden girl to understand your commands.

I would not discourage her love of tennis balls over this. I would simply make sure she knows when you say 'leave it', she is not to go after it. The tennis ball, since she loves it so much, is probably your best motivator in training her. Use it too your advantage. Most people use food as rewards for training but for my Dusty (a 4 year old Golden boy) his obsession with Frisbee's is my motivator in training him. Your's can be the tennis ball, once you get past this hurdle. If you work diligently for 15 minutes a day for several weeks on the "leave it" command, I think you will realize she still had some learning to do!