Terminating parents rights?

i am looking to terminate my rights as a parent. i have custody of 2 of my sons, due to mothers drug addiction and criminal record.the third and oldest child's mother is a bear to deal with on any level. and even went so far as to tell me she had an abortion, miscarried and tried to have me arrested for domestic violence witch did go to trial, and i was found innocent by 12 jurors. she claims that the children i do have custody of beat up, pick on, and talk poorly to our son (the kids i have custody of are 2 and 4) my oldest is 6. she claims that i am the 2 kids mothers problem because of her recent activities. this is just back ground. what do i do? i have come to this conclusion after years of battling with her and to no avail. i'm at a loss and have this current thought that is my last option. any advice would be help full this is a tough decision