Friends of mine rescued a Boston Terrier named Bridgett. She had a number of health issues that had to be treated and a broken leg as a puppy had left her with very little self confidence.
A good holistic diet helped her behavior, as did Bach Rescue Remedy. But she still preferred to hide under furniture, never really participating in the wonderful things her new life had to offer.
Bridgett was 'conditioned' by the people caring for her while her broken leg mended to believe she should remain hidden and protected.
She needed confidence building and one of my recommendations is my favorite confidence building game which I call 'Find It'.
Take a favorite treat, set it a few inches away from the dog and ask him/her to 'find it'. Praise him when he goes to it and reward him with the treat. Each time, move the treat farther away until finally it can be hidden out of sight (under a table or a pillow for example) and the dog has to really work for his reward.
This develops confidence as it is the urban equivalent of hunting down and securing a food kill, it appeals to his basic instincts and makes him feel like a viable member of the family pack unit and not a passive pup meant to stay at all times hidden from sight in his den.
I have used this successfully with a number of dogs and one of the keys to success with this game is a lot of praise when the treat is found.
My Standard Poodle so loved this excercise that in time I could hide a treat really well and keep him occupied looking for it in the house for as long as 30 minutes!