parental rights after divorce in wisconsin

the court awarded me joint custody of my 5 children 8 years ago. my ex took placement of the two younger children and i was allowed the 3 older children. my ex would not allow any contact in these 7 years. i have returned to court many times, but, my ex continues to get her way. she hides the two younger ones from me, has moved to parts unknown, i don't even have a telephone number. my ex has all that information about me. i was awarded visitation every other weekend and she didn't want child support. i would have paid child support - however, with me having 3 children and her 2, she would have paid also. that is why she said no. after i remarried,7 years ago, i have not been allowed to see or talk to my younger children.i have tried. even sent registered mailings. my ex told me that i would never see them again. then she moved and got an unlisted phone number. because i have no address, the three older children could not be in contact with the two younger ones either. the last of the older children are about to turn 18 and be on her own. my ex sent a message via a family member that when this happens, she will take me back to court for child support of the two younger children. i am never going to be able to be a part of the two younger childrens life or part of the decisions in their life. my ex is too jealous and knows the system too well.i now have a bad credit rating because of spending so much money trying to get the court to make my ex allow visitation with the children and failing, i would like to know what i have to do to give up my parental rights to the two younger children and stop the pain. i do not want to cause them anymore hardships in their life and can only hope that some day they will seek me out. my ex has used ways of falsifiying sexual assualt charges on the children just to keep the children from each other and me from the two that she has. we went to court and proved they were false - but it caused alot of pain to the children. just don't know what to do. the only thing left that i can think of is to give up my parental rights.
we are both in wisconsin. she can get free lawyers - i can't.