You need to contact the Texas Attorney General immediately and provide proof that your child is living with you (school and doctor records, etc). They should wipe out the arrearages but they cannot change custody for you. The Texas Family Code provides for an offset if you have actual control and support of the child. They may be able to start a child support review process to stop the support. That's the best free option out there. Use your tax refund to hire an attorney in Abilene who is here on Avvo who can help you. Good luck.
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You may also be able to contact legal aid in your area to see if there are any pro bono attorneys. Although you have had actual care and custody of the child, you need records and evidence to support that situation. Without evidence, too often it becomes a he-said she-said situation and the back support obligation is left standing. The Attorney General's office can help, but it is not a quick route and you want to get started now. Don't delay because the arrearage will only build and the mother may decide she wants to pursue that money at some point.
This response does not constitute legal advice. The facts and circumstances of any particular situation are unique and require an individual consultation before relying upon the general information contained here. The responding attorney welcomes the opportunity to consult with new clients to determine the appropriate direction and strategy for their case.