What can I do legally about getting owed money from a roommate that was not on the lease?

I moved in with 2 other people and signed a lease. We then added another person in to the house with out the landlords knowledge. Rent was split so we paid 180 each. I then paid my way out of the lease, but when I did I had to pay the full 300 because our landlord did not know she was living with us. She refuses to pay me the money back, even with an installment type plan saying "You chose not to stay why should I have to pay for it?" Also, we were budgeted on our gas bill and since no one had good credit it went in to my name. Because I turned it off when I paid my way out she refuses to help me pay that back as well. We only had three more months left of the lease it's not like we had a year to go! Is there anything I can do legally? Or because she was not on the lease do I have no rights?