Your conversation does indeed sound frustrating.
I remember a sign in the school I used to work in. There was a small circle with the words, "Bang head here." It is indeed sometimes like coming up against a brick wall. I am sure that if your head hit once, you wouldn't look to do it again. Therefore, next time, just uh huh mom and go on to something else. I am sure it's because she feels that she is getting older and is desperate to see you "get right with Jehovah" before she dies.
My parents are both gone. I can guarantee you that it just happens so fast. Don't get sucked into another time killing discussion. Tell mom you must agree to disagree, then start asking questions about your family history, talk about things from when you were a child, etc., because after she's gone you'll want to ask but can't.