how to help my pet?

She could just have been under socialized as a puppy. Since you got her from a shelter, her previous home could have provided some negative experiences for her, or she didn't have a good experience at the shelter. Being around a lot of other (loud) dogs as a puppy, could have made her afraid of other dogs. Her behavior isn't quite normal, so I would see your vet just to be safe.

Sounds like under-socialization and maybe some anxiety/confidence problems :/ I would check out Victoria Stilwell's training methods! A lot of people like Cesar Milan but he's very physical with dogs and with a puppy like yours that is very shy and has bitten before (which it sounds to me is a fear behaviour), physical is not what you want - if you push her around she's only going to flinch away more and it will probably just make her scared/prone to shutting down. Positive training and positive reinforcement will help boost her confidence. Some dogs get a confidence boost from doing things like agility too, which help to bond you with your dog as well!! Victoria Stilwell has a website with all kinds of advice. She's put out a couple of books on positive reinforcement training and has a TV series as well as many other resources. You're going to have to be very very patient and upbeat with her - do not push her around or punish her for this behaviour, since it's more likely to make her more afraid.