My 1yr 1mth old GSD has developed some serious aggression towards other dogs. She has not been around many other dogs, but when I walk her and she encounters them it always quickly turns into aggression on her part.
Sounds like a reactive and dog aggressive female. What kind of socialization did she get as a youngster? Regardless of her socialization, it sounds like she is reactive by nature. You will need to control and manage her. No dog parks, no off leash play with other dogs, and maintain utmost control when around other dogs. Keep her away for now until you get control over her. Think about what you are doing and how it feels for her - she sees a trigger (dog), you increase leash pressure, restrain her, build her drive and anticipation, she goes even more nuts, and the drive to act out increases. Quick pop on the leash, good correction, and turn her away. Do not prolong or exacerbate the situation. Some dogs are not cut out to be social with other dogs - don't know if that is her case, but for now, manage her until you can up the training and get a formal evaluation. We cannot see the dog so we cannot offer you the same aid a trainer in person can.
What type of training are you doing? Do you have a trainer? Have you attended classes? How many times a week are you training? What does your training consist of? What method do you use? How has this been working?
She is terribly excitable, which I largely attribute to her still being a puppy- right or wrong. She loves every morning, because she expects a walk. She loves it when one of us comes home and jumps all over us. She loves playing frisbee.
What is her exercise regimen? Whatever she does, double it. Walks are not enough for most dogs. Try hiking, swimming, dog sports for mental outlets, two ball, biking with her alongside. A tired dog is more compliant. Add more obedience and mental tasks to her day - sometimes it is not an issue of physical exercise, but the brain needs to be worn out.
When I went to pick her out I was quite concerned at the mother because she was making circles in her pin that I describe as terribly anxious. I tried to get her mother to stop doing circles, but she just kept going and going. I almost passed on her litter for this reason, but how cute these puppies are.
Sorry to say this after the fact, but I would have left the second I saw the mother behaving like that. Spinning, chasing tails, repetitive compulsive behaviors are VERY indicative of the mental health and well-being of the dog. She was stressed, anxious, bored (?), compulsive, reactive, and threatened by you being there. Spinning to reduce anxiety - a primitive coping mechanism. Especially when this is the mother and a breeding female - very worrisome to say the least... What is she teaching the pups and what is she genetically passing onto her pups when she behaves like that - not at all reassuring. This offers further support that this dog lacks certain traits that would otherwise allow her to integrate as a well-adjusted and stable member of your family - you will need GREATER work and management for this dog. What else do you know about this breeding pair? What was the male like? What health and temperament tests were performed on the breeding dogs? What support and advice has this breeder given you? Do you have contact with any littermates - what are they like?
One last thing about her. She does really well learning with treats, but if something else is going on she will completely lose attention on the training and is unable to listen or control herself. For example, she knows sit, but if we are out for a walk I must always give her a firm push to her back side to accomplish this.
You must be 100% reliable on commands and behavior BEFORE you add external stresses and triggers. Meaning she must be reliable BEFORE you take her out into the world and proof her with distractions. This dog is not ready to follow your directions when faced with something more exciting - she does not have focus or engagement with you. This is a training issue, genetics issues, and some of it is because she is a pup. It is up to you to boost the training and increase practice. Multiple short 5 min sessions throughout the day. Once she is reliable in your home, add distractions, slowly build you - a good trainer used to large working breeds will help.