Having been determined to be disabled probably qualifies as a "substantial change of circumstances" that would permit you to file a motion with the county court for a modification in child support (just the fact that it's been more than three years since your child support order was first entered is enough by itself). You can get the forms for filing that motion from the court house, or you can hire an attorney to do it for you. Whether you need an attorney or not is up to you.
After you file a motion for modification for support, the court will set a hearing date and you and your former spouse will be asked to bring in your financial information (tax returns for a couple of years, as well as a couple months of pay stubs). The state child support attorney and the commissioner will review those, and will determine how much child support is appropriate. That number is going to depend on your gross earnings when the child support order was entered, your gross earnings now, and your former spouses gross earnings. If you are earning less now than you were when the original support order was entered and your spouse is making the same amount or more, you can probably expect to pay less. There is a formula that the court will use to calculate support amounts and unless there is something really unfair with the results of the calculation, the court must use that result.
