My eight month old boxer puppy is afraid off everything and won't listen-any suggestions?

I got my boxer puppy when she was 3 months old and she's been a little scaredy cat since day one (she's now almost nine months). She runs from everyone, she won't go outside and when I do get her out there she"ll bolt back for the dog door or scratch at it till we let her in. She hasn't been abused and we try to give her plenty of attention but she still runs and hides at the sound of anything and everything. When you call for her to "come" she runs to her bed and cowers. She's house trained for the most part but its just getting her to go outside and stay outside that's the hard part. Anyone have any training suggestions? we've tried dog treats too with no success.