If you want her to be able to play with other dogs, maybe finding a mature, well-behaved dog to be around might help? Dog parks are really unpredictable, and can be dangerous for your dog. It's possible she's feeling overwhelmed or ganged-up on if there's lots of other dogs around. I'd only let her interact with safe dogs in a more controlled setting.
Having your dog play with others is not necessary to their happiness, but if she really enjoyed it before (my pup loves it too), then controlling the environment more and finding some safe dog pals to play with is what I would do.
You need to protect your dog first though, so I'd definitely stop the dog parks.
Warden - Sable GSD 4/23/2013
Blackjack - Black and tan GSD 4/23/11-6/1/2012
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