My 18 month old German Shepherd struggles to focus when away from home, every noise or movement distracts her and she is overwhelmed by the environment.

There's a huge difference between playing and training at home and out in the world in a highly distracting environment.

Have you asked your trainer how to work through this? If so, what was the advice? I don't know your dog or your training skill and your regular trainer would be the first resource I would recommend.

I do short engagement sessions with puppies or new dogs or nervous dogs, by short I mean a minute or less. If I can't get and keep my dog's attention for 60 seconds in a new environment then I'm not going to be able to have a training session. Withholding a reward from a stressed dog that is hiding under a table isn't going to teach them that the safe and fun place is with you. I would do these sessions by the car, get engagement and end my session and be done. Do this over and over until you can go 10 steps from the car, 20 steps and so on. You can't reduce anxiety by letting her hide, it will only reinforce to her that she's "on her own" (I hope that makes sense) You can only build confidence by lots of reps of fun and interactive short sessions with you. Many people train much too long and try to make the dog interested when the dog is clearly in the wrong mental state.

Keep in mind some dogs are genetically anxious or nervous and while you can often improve their behavior through thoughtful training the genetics of the dog can't be changed and in a new or unexpected situation there can be setbacks.