I have rehabilitated some dogs like that by fostering them with my gentle, kind-hearted, peaceful female, who is amazing at reading other dogs and deciding when/how to engage them without frightening or antagonizing them. I follow her lead on what they need, as she can read them expertly and intuitively knows how to help. When she hangs back, it means it's too soon -- she'll know when they're ready. I could not do half of what I do in rescue without her. She teaches them what I cannot. You need to find someone with a dog like that.
I don't know how to tell you to find that kind of dog--she was sent to me as a gift from Upstairs. I just know that kind of dog can work wonders in helping the kind of issue you describe.
I also agree that a good beginner OB class can do wonders for a dog's self-confidence, if you go about it right, with that as your goal. I've seen a few dogs completely transformed about 3 weeks in, when they realized they were totally safe around other dogs in the class, and they always knew the right thing to do and got praised for it because they were set up to succeed. It's a beautiful thing when you see that happen--the posture changes, and the whole affect is different in the blink of an eye, when their world suddenly starts making sense and they totally trust you. I would go into the class making building the dog's confidence the goal, with the skills just being a nice side-benefit (and convey that to the trainer--a good one will be very supportive of that).
Last edited by Magwart; 11-09-2015 at 06:11 PM.