First, good on ya for rescuing this girl, and for having the patience and commitment to help her overcome the shadows of her past.
Second, in behavior rehab terms 9 months is but the blink of an eye. So don't be discouraged, it really still is early days. I know how this can get you down - been there, am still doing that - but true rehab, changing the emotion that drives the behavior, is a slow process.
The horrors of a dog's past can cast a long shadow - we need to be aware of the depth of damage previous bad experiences can cause, and be patient. The Hooligan and I are five years down that road... with patience, understanding, love, time, more patience and more time - it will get better.
I have learned to break down my dogs' behavior problems into small goals, as the overall picture can be overwhelming. So rather than tackling 'reactivity to other dogs', I set my goal as 'today we are going to walk calmly past the yellow house where Fido lives'. And then I celebrate our 'victory' when we manage to do that. As the owner of a dog in rehab I know: you need a pat on the back just as often as your dog does.
The second thing that has helped is that I follow a two-stage process - first managing the environment, then rehab. Because rehab is a long term activity, in the early days I try to manage my dog's environment so that she does not have opportunities to practice, and thus further ingrain, the behavior I want to change. If that means a somewhat restricted environment at this stage so be it - safety first. If you give up some concrete examples of flash points that you are having trouble dealing with then perhaps we can suggest some short term management strategies.
And third - take time to enjoy your dog, regardless of what else is happening. When things are not going well with the Extreme Belltie I take a break from rehab work - we spend the day just cuddling, taking it easy. From time to time I need to remind myself why I'm doing this.
You'll have plently of down days, lots of one step forward, one (or two) steps back - that is to be expected. Then suddenly something will click, and you two will make a leap of progress together - it's those days that make it all worthwhile.
What you describe on the farm does not surprise me - behavior is often situtational. For instance, my reactive mutt is more relaxed at our Hundeschule, things that would send her into the red zone in the big wide world she takes in stride there, as she has learned that the training ground is a safe environment. By working on her behavior first within her comfort zone we set the building blocks that enable her to be successful in a more stressful environment.
Is there anyway that you could work with your dog more on the farm grounds?
I am hesitant to make specific suggestions - one cannot get a true picture of what is happening without seeing a dog, as the wrong approach can do far more harm, especially with a GSD - the breed is extremely sensitive. Also, I am not a behaviorist, only an owner with of a herd of screwed-up mutts who has therefore had the fun of gaining some experience with rehab. You would be best served by working with a good behaviorist, one who has experience with similar problems.
Could you travel to southern Germany? If so, you might try:
http://www.animal-learn.de/
Clarissa von Reinhart is a very respected behaviorist. She is the author of 'Stress in Dogs', a classic on the subject, btw. Even if that is too far away you might contact her asking for suggestions.
Wishing you and your dog all the very best.