You may file a Supplemental Petition for Modification, to modify the payment. A modification requires a substantial change in circumstances (something that is unforeseen and a substantial change from when the order was last established to your current situation). Check to be sure that the payments you are making are actually going toward the arrears, and are not still considered child support payments. If the latter is true, your supplemental petition would simply alledge that the children have reached the age of majority. Then, during the process, an arrearage payment will be ordered based on your current ability to pay.
This answer is provided for a general purpose, as I am not privvy to all the details or documents relating to your case. This answer does not establish an attorney-client relationship. Prior to taking any action, you should consult with an attorney who has the opportunity to learn all the factors of your case or situation.