Hello
I love the 80s too! I am also glad to meet you and sorry for the circumstances. As a parent of an adult child (my 24 year old son) with BPD I know what you mean about circular arguments that go nowhere. I find nthe best thing to do is back off and revisit my own boundaries. Here is a great article about boundaries that really helped me.
Healthy Boundaries Here is an excerpt "Know your priorities. Don't get caught up in dramas (conflict where one or both parties want to "win the fight" and have lost sight of solutions).
Try to collaborate and get a buy-in on solutions. Be prepared to take unilateral action if that fails.
Remember, the goal is not to fight or to argue what is right and wrong in an ideal world, or to have it your way, or to control others.
The goal is to live true to your values (1) and the dependent values of the relationship (2)."