The problem, as I see it, is your dog has no confidence in you as his/her pack leader. Our pets, especially dogs, look to us for the same protection, confidence and leadership as they would do in the wild. from the Alpha male. (You needen't be male to achieve the same results). They operate strictly by instinct, no matter how "civilized" we have attempted to make them.
If your self image is one of confidence and calm assertiveness the dog is going to pick up on that and be more reliant on your ability to handle threats, even ones from other dogs. It doesn't take much fear on your part for the dog to instinctively sense that and go into the flight mode...
Walk the dog on a leash with your shoulders back and a purposeful stride. The dog should never be straining the leash in front of you, but should be at your side. When other dogs approach and your dog begins to display nervousness, a quick flick of the wrist on the lead will check that impulse and recenter your dogs attention on you. In fact, you should notice the dog looking at you, especially your face to determine what he should do.
If you need to, stop, address the dog directly, placing your hand in front of you, palm up and look your dog directly in the eye saying firmly but not loudly "Stop - sit". The need is to keep the dog's attention focused on you, the pack leader. If you'll do this consistently and with purpose, you will notice a difference in the dog...
Best of luck...