Hi Claudia65 and faithfuldog!
Here is a copy of my posting in the past that explains how we worked on Noah's fear aggression towards big dogs.
Lead aggression
Noah still has leash aggression and we are still working on it. He is much better now off leash since he learned how to ignore or just come back to me when he sees big dogs at the dog park. However, I don't want to recommend you take your dog to a dog park because it is like flooding your fearful dog with other dogs. I think it will work better if you let your dog meet a trustworthy dog, one at a time.
Noah is now having no problems with my foster dogs (so far I brought 5 different dogs home - all 35 to 55 lbs) and he has even learned to let his ball go when the foster dog tries to take if from his mouth. If it was until 8 months ago, he would have had bit the dog. It took almost one year for him and he is not perfect but it DOES get better if you are consistent and keep working on it just like taniajannifer said. There is no quick fix so please be patient.
Good luck
Hi taniajennifer! I am so glad to hear that Libby got over the fear! You did a great job!!