@Hello1989 well first of all, I'm sorry you are having this stress.
However, you've given me a perspective I possibly hadn't appreciated.
My child is an only child, unlike you and your Mum, we have a very good relationship, to the point I share a home with child and their partner.
Like your Mum, I will, on occasion, talk about my death. I'm 75 and I'm not in the best of health. my death is inevitable, and I treat it as such. So I'll say things like "I hope you'll wear this watch when I die" or "I'll ask my sister if she'd like this ring when I die". My child always rolls their eyes and shuts the conversation down.
until I read your post, I had put down their reluctance to accept the inevitable. Maybe, like you, my adult child is stressed already and I'm adding to it.
so thanks for your perspective.