http://www.healthandgoodness.com/hap...self-worth.htm
This article might help you. A person usually defines themselves by their past experiences. Your past experiences have all been negative. That being said you help others because it makes you feel good, something that you don't get to feel too often. You've been probably been taking care of your dad since the day you can remember, however no one has been taking care of you and it has finally starting to catch up.
What are your dreams? If you could do anything in this world, what would it be? How would you define happiness? What do you enjoy doing?
We can't change the past, wish we could. All we can do is allow it to make us a stronger person. You also can't define your own self-worth by what others have done to you. You did not deserve to have been mistreated and nothing you did or said caused it. They are just bad people who unfortunately crossed lives with you. Not to say you won't run into bad people again, but what you can do is start concentrating on what you want and how you can change your future for the best.
Have you also looked into individual counseling? That might help you as well.