Lhasa Apso Q&A
I also have an AB and we had a similar problem. I took mine to several obedience classes and made sure she was socialized when she was a puppy. My dog was about 6 months old and I was walking her - we were surrounded by 3 pack dogs and she tried to defend me. The other dogs live across the street and one day she went over looking for them and stuck her head into their fenced area and they attacked her. She was pretty torn up, but ended up with just stitches. We built her a bigger sturdier fence to keep her in our yard and a boxer brother to keep her company. To this day three years later she still watches over there from her side of the fence looking for those dogs. I don't think you can ever get rid of that.
It happened to my female also but she already had a good relationship with a variety of brothers and sisters. She has never forgiven those dogs however. I think this breed is too intelligent to forget and forgive. If possible, I would try putting a non-reactive dog on the other side of a fence (in safety) and let him gradually get used to it. Good luck..