Hi byfaith,
I can relate with your post. I set boundaries after the marriage was over so it's probably harder for you because you're in the r/s. I was so worried about the backlash.
Since we shared a business together and property, I had no idea how I was going to unravel the knot that bound us, but slowly, bit by bit, I untwisted it and thank goodness I did
It gradually gets easier until it becomes second nature and you don't worry as much about the other person, boundaries is really about self compassion, self care and protection.
The thought of establishing a particular boundary brought with it thoughts of being verbally degraded and belittled
This is most likely lashing out because the person is not used to it, it'll die down. You probably already know this but another boundary is to step out of the house if you're getting verbally attacked.