It sounds like a lot of the problems you are having with your dog are the result of too much off leash, group play. Not all dogs are suitable for group play and for many, it is too overstimulating and triggers other undesirable behaviors, such as on leash aggression and predatory behavior towards small dogs. GSDs are generally not the sort of dog who are ideal for group play situations and with any dog, allowing dogs to become too interested in other dogs is going to lead to some training problems.
Before using an ecollar, I would suggest that you stop taking your dog to group play and spend at least 8 weeks focusing on very strict NILIF. Treat your dog like she was a young, untrustworthy puppy again in the house. If she isn't interacting with you confine her to the room your in or crate her for awhile with a food puzzle toy. No free playtime outside. No off leash play without a long line. She has to "ask" for things she wants by sitting and looking at you (Want to go outside? Sit and look. Want dinner? Sit and look. Want me a toy thrown? Sit and look. Want in the house? Sit and look.). Have several short training sessions a day. In at least one of them, work on practical stuff like sit/down, stay and in at least one of them work on fun stuff like tricks. Aim to teach her at least one new behavior a week, using nothing but treat training which will be fun and bonding for both of you. Commit to giving her at least 45 minutes of interactive exercise every day. This means exercise which involves you interacting with her - fetching, hiking, walking, weight pulling (requires a harness and with a young pup, just having her pull some light weight objects around the yard), agility training (can use homemade obstacles but again with a young pup, keep jump heights low), etc. Work on a recall program such as this one, starting at the beginning: http://www.shirleychong.com/keepers/Lesson6.html
Focus on impulse control work in your daily training. "It's Yer Choice" and "Crate Games" are a great way to do this:
Crate Games Part One:
Crate Games Part Two:
Crate Games Part Three:
It's Yer Choice
Eight weeks of this sort of training program should show you a lot of improvement with your dog, providing you stick with it. While e-collars can be effective for some situations, they are also a tool that has the potential to "break" your dog, causing them to be nervous, skittish, phobic, fearful, withdrawn or aggressive. I have known dogs who were changed for the worse when their owners opted to use ecollars for certain behavioral problems. Ecollars are absolutely not a tool you should try buy and attempt to properly use without professionally guidance. And even with professional guidance, make sure you talk to as many clients as possible and see how their dogs behave. If you see a lot of stressed out, "shut down" dogs, consider if that is how you want your dog to behave. Never allow a trainer to continue using any correction (ecollar or otherwise) on your dog that makes you uncomfortable and make sure you are present for all training sessions.