Being sued for a debt I cannot pay

I am being sued by a debt collector. I had no knowledge of the lawsuit until I received the summons in the mail. I answered the summons, but at that time I didn't know about the affirmative defense. Later, I went to court for an administrative hearing, but the debt collector never showed up. Now I have received a judgment summary to go to court on July 5th. I am writing an affidavit to dismiss and want to include my affirmative defenses, but I don't know if it's too late to do so. I sent a letter to validate the debt and they sent me copies of statements showing I did make payments to bank of america. I did take out a loan with bank of america, but the amount the collector is asking for I do not owe. I borrowed $15,000 and paid back $10,500, but they say I owe almost $20,000. Please, what else can i do? I am unemployed and have not worked in 2 1/2 years. Can I add the affirmative defense to my affidavit to dismiss? Hope to hear from you soon.