When I went through the same thing, what ultimately gave me the strength to go no contact with my birth mother and most of the assholes from that side of the divorce was when my psychiatrist 'gave me permission' to go on ahead and do it.
It wasn't so much that he permitted me, but that he asked 'what do you owe these people who have given you nothing but pain?' But having the exterior affirmation/confirmation that those people were actually assholes, and my pain was legitimate was empowering.
I give you permission to cut that cruel asshole out of your life. Ignore those interior voices that insist that you owe them anything. My son will never know his maternal grandmother, but that is entirely her own fault, and my stepmother is the best grandmother a kid could hope for, and I consider her my mother at this point.
It's still hard sometimes, and those echoe-y guilty feelings pop up every once in a while, but I am a much mentally healthy a person for having done it, even if I am on a bit of a downturn at the moment.