She is not being defiant. She sounds like a delightfully wild puppy.
I am just responding so this will show up in my tracked threads. No time to respond in depth right now but I am certain you can get the help you need here. You do need a trainer used to working with these kind of dogs. You probably are in over your head but a good trainer will throw you a lifeline.
I will say at this age and size you may need a prong for control but her training needs to be done at a distance from other dogs where she is not so distracted and then ease her into distracting situations..winter has probably put a major kink into that. One on one may be best for you right now then work towards bringing her into a class. Can you play fetch out in the snow? Two miles of walking and ball in the house is probably not enough exercise. I never play ball in the house or anything wild because to me inside=calm and they don't understand if they play inside why they can't be that way all the time.
NAPWDA Certified Cadaver Dog
Waiting at the Bridge (italics
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