What can I do if I'm not the biological father but I raised the child for the first 7 years and now they are taking me for child support?

Little bit of background, I was with a girl she got pregnant and told me the baby was mine. After she had the child, I signed the affidavit of parentage. The girl and I split, I took her to court for custody. She was abusing drugs and wasn't a fit mother. The court awarded me custody. In the long drawn out court case, she wound up telling me that the baby wasn't mine. I thought at first she was just trying to hurt me and get me to stop fighting for custody. I come to find out that wasn't the case and she was telling the truth. Skipping ahead to now, I no longer have custody. Another family member of mine has custody. I got in a bit of trouble and was in jail for 6 months. The family member and I do not talk at all. I've seen the child maybe 15 minutes in 2 years. In the 7 years that my wife and I had the child, we never received any help or support from the biological mother or anyone else for that matter. I stepped up when nobody else could when he was younger and now that he’s older, they want me to pay child support for a child I took care of because nobody else would for the next 11 years. I just don't know what paper work to file, where to begin, or if there's even a chance that I could do anything about all this. I can't afford the child support they have me paying now and I'm not willing to lose everything I have over all of this.