Welcome, wildflower11!
Just wanted to pop in to join
Mutt in welcoming you to the forums. I'm sorry for what you're going through but glad you have found a community where many of us have been through similar experiences, and we can learn from each other. I wish for you as much help and support as I have received.
Thank you for what you have shared with us thus far:
How do you reconcile firm boundaries with a desire to help them and be compassionate?
I imagine that interacting with your mother is very challenging, to say the least. You are a very compassionate person and that shines through in your post. It also sounds like you have learned how to take care of yourself very well by setting boundaries with your mother.
I am finding it very difficult as I wonder each time with talk/see each other if its the last time.
This is a toughie and makes total sense. I bet this is difficult for you. But, of course, in order to take of yourself, you can't very well let her run all over you, despite the fact that she has terminal cancer. It sounds like you are already doing the very best that you can, and that's all we can do, really.
I know she isn't going to change, but I would like to have our last interactions be as positive as they can be... .and her interactions with my kids too (age 8 and 5).
Well, you're right that if your mother suffers from BPD, I doubt there's enough time left in her life to change. So, in order to "keep the peace" while honoring her last days, the onus will have to be on her family members to set clear boundaries and possibly step back from the fray (if necessary), while also engaging with her in as positive a manner as possible.
Have you seen some of the material located in the right-handed panel?

The site articles may help provide additional support to you while you manage this very difficult time.
I'm sorry you're having such a tough time, but we're so glad you're here. Keep reading and posting, and I'm sure you'll get help and support you need (as I have).
Keep writing, keep processing, keep learning!
-Speck