Do you think your anxiety is related specifically to the age gap? Or do you think it could be deeper and that it's possible there's other anxiety about the age gap that is just coming across as an anxiety over him dying before you?

The reality of the situation is that either of you could die at any moment, regardless of your age, from a variety of causes. That probably doesn't make you feel any better or improve your level of anxiety, but that's why I'm wondering if there's something more to this age gap anxiety.

I'm sorry I don't have any advice to help you, but I agree. Try to stay mindful and in the present so that you can enjoy your time together rather than spend it stuck in anxiety. I know that is so much easier said than done.