Hello,
As with any fear associated behavior it is best to start slow and gentle and begin to gradually reassure them that things are ok. In some cases I use treats, a short lead or even a dog bed to place a good reward with conditioning out a fearful reaction. Things like placing her in a crate she is comfortable in to relax her near the threshold and then walking slowly outside and giving her treats as you go or as she approaches you. Often you will find there is a specific point that they won’t advance past. This is the time to sit, be calm and gentle and reassure her that things are ok. Never do anything too fast, and don’t force her. You have to be calm and gentle so she learns trust. Don’t yell and never hit even if you are frustrated. I also think that electronic fences are not ideal for the reasons you have learned. Try leash walking and building trust and then start conditioning her out of her fears. Try also looking at Victoria Stilwells stuff. She’s great! Google search her name for lots more training info.
There are many factors that can lead up to this type of situation. Have you discussed this with your vet? Have you spoken to your trainer about it or a behaviorist? I’m not sure of your daily routine with your dog as far as walks and training, but those are absolutely KEY in keeping your pack order straight. I would start out talking with my vet and a trainer and take things from there. There is no such thing as a perfec pet. They all require time, work, lots of love and patience. No matter how docile they seem to be, anything with teeth can and will bite, unless properly trained and exercised and knows that they are a part of the pack. Best of luck! Keep us posted.