Leasing Problem! Please help!?

In August 2009 I started and signed a 12 month lease with a college apartment complex in Waterloo, IA. In December I decided to move out, because I couldn’t find a job or afford the rent. I went into their main office and told them exactly that. They said I would have to find a subleaser and that was my only option or I would have to pay for the remaining 7 months. I couldn’t find anyone to lease for me and finally just moved out. I explained the situation to the owner and she said that they would look for a subleaser for me. I still had 7 months left on my lease, so I told them that if they looked for me I would pay for 3 months of rent ($1,125) and they agreed. I just got a bill for $1,600 for unpaid rent and damage to the apartment I didn’t live at. What should I do and how should I address the situation?