I am so sorry Plath, these conversations seem to take the same path every time we have them don't they.
My therapist and I have this conversation every week, you know what your mother is going to say and you know what you are going to say, and is anything going to change when the two of you do this dance every time you get together?
NO NO NO NO and NO! Everything ends up the same.
It makes me just as crazy as it does you, the only thing that works for me is to avoid my mother or avoid these conversations.
I have the same problem with my father on religion, now when he starts in I just say, "we have had this conversation 100 times and nobody's mind will be changed if we have it 100 more times, so we are going to have to agree to disagree" if he continues I just refuse to discuss it with him.
You cannot help that the pass was closed, her being upset about it is unreasonable, think of her like a small child, handle it the same way.
I wish I had better advice, but you cannot overcome denial and unreasonable expectations, and you cannot fix stupid.
Big hugs
Bill