
Welcome Cerasoli,
It was hard to read your post, you are going through so much pain. I'm sorry for that. I'm a grandma too, I'd be right there with you watching and listening to everything he did. Obviously something was effecting him, you saw it. My mind goes o the statement you made "I admit I am afraid of her". How sad that is and how much pain that must cause you. But if that's how you feel, I would wonder how her feels about his mother.
I do get it, I know that is just the tip, my daughter is 35. Man can she be vicious I don't want to be around her during those times either.
It sounds like you could benefit from using the S.E.T. method.

Find a lesson here to the right under tools ---------------------------------------------------------------------------------------->> >> >>. When the way we interact with the BPD is not working, we need to need to change the way we do it.
I can tell you love your grandchild and don't want to up set the balance of being able to spend time with him. And at the same time I hear the weariness in you at dealing with your daughter. You asked the question "does anyone think I should just let her go"? I cannot answer that question but, I can tell you that sometimes when we as parents are so worn out, a break is a good thing; for us!
Some quiet can always be refreshing. You have so much on your plate and you always bite your tongue to keep the peace. What about you? What about what you need? There are plenty o other parents here who feel just like you do right now. Take your time, read some other posts.
there is great support here for you.