We have an 8 month old female GSD. She is lovely off the leash, plays wonderfully with dogs and is great with people, but as soon as we put her on the leash she is lunging and barking very aggressively towards other dogs. It's really awful. If a dog does come up to her she cowers away. We are thinking of getting some help for this (v expensive....).. any advice would be massively appreciated as I'm getting worried she is going to get aggressive eventually.