How can I stop paying child support when I now have custody?

If you have always paid the full amount you are court ordered to pay then you should file a motion to modify the order and have it ceased. If you paid less than the full amount ordered while your child was in state custody the state may be recovering past arrears for which you have no remedy.

You need an attorney to get you to court and file motions on your behalf to a) seek reimbursement for those monies you should not have been paying to the mother when she did not have custody and b) to seek a full amendment to custody order so that you don't pay anymore and in fact, file motions to make her pay you. Ultimately, the money is for the benefit of the child and if you have custody, I don't see how the court will not approve your motion.