Very nervous dog

Question
I have a wonderful Sprocker. She is 2 years old and I have had her for the past 9 months. She is very obedient and rarely barks, I have never heard her growl and never seen her snap. At home she is the perfect dog. However, when we go out for her exercise she is fine until she meets another dog, then she tries to hide behind me, she seems terrified of them. If they come close and try to sniff her, she puts her tail between her legs and slopes away, close to the ground, if she is off the lead. One barked very close to her recently and she yelped, even though it didn't actually touch her. Unfortunately she is exactly the same with children and appears very frightened of them, tries to hide or run away. It seems that when she gets to know someone she is very friendly, but with strangers she will try to hide. I'm afraid that one day a child will try to pet her and if they continue and ignore her obvious reluctance she will turn and bite. When there are no dogs or people near her she is confident and runs around, chasing birds and squirrels (her favorite chase game), on the beach she will run in the sea and on the beach.

I've enquired about her past but can't find any reason why she would be like this. She was bred as a gun dog by a gamekeeper but wasn't deemed suitable as she is afraid of gunfire and wouldn't retrieve.

Any suggestions please?

Answer
On walks, when you see another dog coming, tell her to sit and stand between her and her view of the other dog.  She doesn't know what to do, so you have to tell her.

Don't take her to dog parks right now.  Start with a friend and her dog.  Humans in the middle and a dog on either side.  You walk...you stop..ignore dogs.  When you see the signal your girl is getting antsy...say "Let's go" and move on.  I've seen dogs become buddies after an hour.
But you need a nice, cool, stable dog to work with.

She obviously wasn't socialized so you're going to have to do it one step at a time...one dog at a time and one child at a time.

If she's nervous when people come in your house, ask them to walk right by her with not even eye contact.  Have her on leash - make her focus on you and what you want her to do.  Train her to sit away from the door.

There's more, of course, but I'd like you to try the 'on walks' suggestion and tell me how she does with that.  Make her sit - give a leash correction, if necessary - stand between her and the strange dog and once the dog has passed..."Let's go".  All done calmly.  Don't chat - soothe - just give the SIT command.
Delores