I'm exactly the same with my mum. She's 85 now and I know she will never change. We though have come to an unspoken truce of not really talking about it anymore. We talk about everything else except this. I have to really bite my tongue a lot, it's very hard for me mentally. We had a lot of arguments in the past, we would both get upset and then not speak for weeks. I visit her every day, and I hate the fact that she will die still believing that one day she will wake up in a paradise.
I hope things get easier for you.