When working with a dog, it is crucial to work within their emotional ability. This is called working sub-threshold or it can be described as at a challenging level but within the dog's ability to be successful. For example, with a fearful dog one would work the dog at a distance from other dogs in which it is challenging for them but not so close that they become reactive. As the dog becomes accustomed to the fearful trigger, he/she pays more attention to their owner, and learns obedience commands, one might be able to slowly shorten the distance between the dog and the stimuli to a new challenging distance.
To determine the distance to be used between the dog and the fearful stimuli, it's important to be able to recognize stress behaviors in dogs. If you see your dog experiencing these behaviors, create greater distance between your dog and the stimuli.
One can play detective by observing what stress behaviors your dog is showing at a particular moment. Or, an easier way is to observe what your dog looks like when he/she is totally relaxed and stress free. How does your dog stand (leans forward/backward/etc.), how tight are his/her muscles, how does he/she hold their tail (held high or tucked under), etc. When you and your dog are in a stressful environment, you'll be able to recognize that your calm stress free dog isn't present and more quickly be able to increase the distance between your dog and the stimuli.