Oldest turning 18

My daughter turns 18 in June a few weeks after she graduates. I also have another child who is 15 that lives with my x as well. What do I need to do to get my child support reduced? I live in Colorado now and they live in NC. I talked with the NC child support enforcement agency and they state that the order just says one amount and they are not willing to file a motion to reduce it.
Also, my ex thinks that I am going to pay for my daughters college, which I had to take out student loans for myself.

You need to register the Order with the court in your county in CO. Then I suggest you contact your local child support agency to request a modification of support. Jurisdiction is proper in the state you live in, not NC.