Losing Someone

Ok I know this might wound really stupid to some people, but I am 31 years old, and don't know what to do when you lose someone you love. Now I don't mean losing your boyfriend, or best friend, I mean losing someone that has been there your whole life, that you really don't but into terms of "one day I might lose them", or at least I don't. I have never lost a grandparent, parent, sibling, aunt, uncle, cousin, or close friend to death. And now my Grandpa Weinfurter is sick, and not doing well, and it seems weird to think he won't be around to see in a year or two. He fought cancer 18 months ago and one, and now it is back and in a different part of his body, and he is just giving up. I am going up to Wisconsin in a couple of weeks to see him before he passes away, but what do you say to someone that you have loved your whole, when they know they are at the end of their live. How do put all that love you feel for them, into words? It's so weird. I just have this silly notion that everyone in my family is going to live as long as I do. It's super cool if I die before them, but they better not die before me. I have been super blessed to have known 3 of my 4 Great Grandmothers (all of them lived into their mid 90's). 2 of them were around for quite a while in my life. And those were hard enough, now how do I say Good-bye I love you to my grandpa? He is 80 years old. If he wants to let go and not be in pain then that is fine by me, but I want him to know that he was loved very much, and that he will be greatly missed. I just don't know how to say that to him.