Hi RD,
Your English seems excellent to me

I'm sorry for what you're going through. To have a one-night stand that ends in 18 years of coparenting with a BPD sufferer is a bitter pill.
What are some of the things she is doing to make your life miserable? Stop Walking on Eggshells is a good place to start. I don't remember if the book describes skills that can help with boundaries. It can be taxing to set up these boundaries, and sometimes it can feel like parenting a child.
You know for certain the child is yours, I assume. How old is your child? What kind of coparenting agreement do you have in place?
Glad you reached out for help. These are difficult relationships to manage on our own. Having others walk with us can make a big difference.
LnL