How do I stop modification of child support?

Father litigated for near 50 per cent. I could do nothing more to stop it. I have been SAHM, for nearly 15 years and still am to the 5 children. He now wants modification since he has more time. The support amount has not increased since 2009. He is in arrears. He doesn't pay when he should. Neither has he paid the full amount. I pay all out of expenses for orthodontist, dentist, etc. He sought this day after I finally was able to move out of my parents’ home into our own home. He is trying to drown me financially. Help, I can no longer afford counsel. I was married to and divorced a controlling manipulative narcissist! He does not stop litigating!