Tough situation. One exercise that might be very useful here is "treat-retreat". In this instance you would sit somewhere quiet with the dog (can also be done just chilling on the couch etc.). Toss a treat a good distance behind your dog (3-4 feet minimum). The dog will likely get the treat and then turn around and look at you, hopefully move forward back to where they were originally. Toss the treat again behind the dog. Repeat a lot ... by doing this we are reinforcing the dog for moving towards you but then removing any pressure by having the treat a further distance. Often this works very quickly and the dog closes the distance between you quickly. If you do any clicker training then that can be useful with this exercise.
There are other variations on this exercise. You can toss a treat and then move backwards. Try to remember to keep your body sideways to the dog as that is less threatening.
If your dog is very comfortable with another in the house then you can look at doing some "BAT" type exercises as well. Start with you sitting in a chair. Dog is on leash with someone it trusts. Have that person walk towards you. The moment the dog shows discomfort it has crossed threshhold. The person wants to stop before that happens. When you stop do a quick treat, good dog and turn around and walk away. Basically you are rewarding any non-reaction and then again removing pressure. With practice the person should be able to close the distance easily and then after that progress to you standing etc.
Remember baby steps! Allow the dog to make choices and things will go fast because the confidence is being built. Good luck.
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