An animal or a child, who is neglected because of the lack of education of its primary caregivers, can be recovered with little to no long term negative effects altering their basic personality. They are cautious but curious and can be taught to trust their environment, once given the opportunity to experience a stable world.
This is a vastly different experience compared to if the neglect is based on malice. This will leave a residue of submissiveness or anger in their personality, both of which are based in fear. It is almost impossible to overcome this obstacle, even after massive re-education and re-training because in times of stress, they revert to fear based reactions.
You have to make a decision on where and how you extend your energy and resources. There are times when you have to recognize that the past which makes us who we are, can result in basic personality faults that we can not overcome, in spite of out best efforts and intentions. Sometimes the hardest thing to do is to walk away, especially when all you want to do is ‘fix’ something what was broken, long before they came into your life.