I'm Doug, and I'm very sorry to hear of your situation. My goal is to provide you with excellent service today.
This seems like a very crucial matter for you, and your questions and issues suggest that an in-depth conversation might best suit your needs. If you are interested, for a very nominal charge of just $9 more I can offer you a personal phone conference during which there is no limitation on the number of questions you can ask of me, as opposed to continuing in this question and answer thread which is searchable and viewable by the public.
Let me know if you don't want a call and I will be happy continue here with an answer for you.
Either way, whether I call you or not, in order to give you a clear and concise answer, I will need some additional information about the circumstances, please.
1. Was your child, born after the divorce, the natural child of your ex?
2. As the youngest child is not yet 18 how do you know you cannot pay the agreed-on amount?
3. When you entered into the agreement, how did you plan to pay it? Have your financial circumstances changed?
4. How many of your children from your ex presently live with you?
Thanks in advance,