>>49828989
What's the problem here? You're romancing a girl with a lifespan that's vastly longer than yours and (hopefully, if the writing's good) explore her feelings toward you, including that. When you capstone that sort of relationship with an ending,
>And they lived happily ever after
is pretty stupid, while
>And they lived happily, and she looked back on the life you shared with fondness
is more thoughtful considering your relationship.