I’m going to keep this one short and sweet. If you are working with a fearful dog you must at the very least understand the concepts of counter conditioning and desensitization.
Desensitization is the gradual introduction and increase in exposure to the things your dog is afraid of. The exposure is only increased when the dog exhibits comfort with the situation or object. Go too fast and you set yourself back by causing a fearful reaction to whatever you’re trying to get the dog used to. This usually requires more time and patience than people give it. When it doesn’t work they blame the concept and not the way they implemented it.
Counter conditioning is the pairing of something the dog is afraid of, with something the dog loves. It’s classical conditioning except that you’re changing a negative association to a positive one. It is done in combination with desensitization. If the dog cannot engage with whatever you’re using to create the positive association, food treats or a toy for example, then you are not counter conditioning. If your dog will refuse to eat a super favorite treat when someone is 4 feet away, then try having the person be 10 feet away and see what happens. The biggest mistake that people usually make is not having the reward be of high enough value, and using enough of it, often enough, to outweigh the negative feeling the dog has for its fear object.