Sorry to hear this,
My dog is fine around others but my bitch isn't,she was attacked by another dog.
Sometimes this will happen anyway,as staffies mature some will not tolerate other dogs.Unfortunatly you will always get numpties with loose out of control dogs with no recall.
I avoid areas where dogs are likely to be off lead,but you can never be sure you won't meet one,the only advise I can give is keep trying, and if you spot an offlead dog shout to the owner to put it onlead till you have passed.

Don't be suprized if they say it's friendly,that's the usual response I get.