How to change the amount of child support that I agreed to??

I was married and divorced at a young age, 1997 to 2000, the reason I state this is #1, I was young & naive and #2. my ex-husband went with the cheapest way to divorce, he had a (paralegal?), draw up what we would agree on and the judge signed it, the problem is the Child Support agreement. I should not have agreed to such a small amount, However I am trying to find out how to go about having the amount increased. When I agreed to this he was not making much money and we were in our twenties with one child, & he was very involved with our daughter at that time, and he would help her & I out when needed, we stayed very close as best friends, so at the time I never considered that one day he might not have anything to do with her, and not fulfill his promises & that is exactly what happened. Approximately 3 yrs. ago he remarried & has completely abandoned our daughter, he has not contacted or even seen her, no birthday card or Christmas presents: (nothing ) and what makes me even more upset is he lives less then 15 minutes away and since this tragedy, this has completely devastated our daughter, she has major depression & anxiety issues now, failing in school, runs away a lot and into drugs and alcohol, very angry all the time and out of control altogether. I do not understand why he cannot be held responsible for these problems that he caused. Our daughter was fine & happy before he destroyed her. Also for about 4 years now I have not been able to work due to a severe accident that has disabled me, I have already had major back and neck surgeries to try and help me, with no avail, I have been and still fighting for my disability. So you see I need all the help I can get. My Ex-husband only pays $200 a month and this is through the state. Though since his new marriage he is never on time and a lot of times he skips a month or two altogether. Also he makes a lot more than he used to. I feel so ignorant and foolish for not realizing.