Do i continue to love or let go?

So my current boyfriend is 20 years older than me, and Idk why but i started to have these thoughts about losing him in the future to natural causes...
I mean when I’m 40 he’ll be 60, should i save myself the pain now and leave before i get too emotionally attached?
I know this might seem irrational and i should live in the moment instead of running out of fear, but that fear is real. The fear of being alone, losing someone to death... someone that close.
But there’s possibilities of losing someone no matter the age, anything could happen.
So what do I do?? How do i ignore these crazy thoughts? Or are they not so crazy to think?