I think you're trying to make your feelings conform to your Nana's expectations out of your love for him, but your mind is telling you otherwise. Stop trying to make an emotional connection to override the truth of what your logical side is telling you. He is not going anywhere, so relax with the romance stuff and concentrate on your own life. Kindly tell him that you're not ready, and you don't want to tie him down, and you don't have any animosity towards him, just the timing is wrong. You're only 18, don't rush.