Where is the justice?

My x payed peanuts for my 2children he caused the family to break up .now he has turned my son against me I haven’t seen him for over 18 months I rang the csa and they told me to stop paying csa as they hadn’t heard anything to say the child in question was in further education so my x got his pay off through the courts so I am now paying a loan in place of child maintenance there is just my wages going in my house they have two very good wages 2 lots of child benefit what they shouldn’t be getting maintanence from her x husband and income on the side .

Then out of the blue gets a phone call4 months later to say I owe back pay and have to pay again there fault not mine where im getting the money from god nose .I have tried ringing the collage because my son goes twice a week not full time and they wont tell me any details im his mother for gods sake.they between them go on holidays .I carnt even afford to buy food I have my daughter living with me.who earns next to nothing .weres the justice,ive been to the C A B ,I am now writing to my MP .I don’t begrudge paying for my son but he doesn’t get it the spend it on drink and going out this is making me ill,i have just come off anti depressons because of the way he as treat me and my kids he tried to kill us and hes got away with everything theres no justice in this word .