Plan a special date---one your girlfriend will enjoy and never forget. THEN, when the right moment comes, with you looking into her eyes, surprise her with ONE unique beautiful colorful flower. Tell her you chose this unique ONE flower to give her as a preserved reminder of this special night---because SHE IS special to your heart.
Everyone overdoes the "dozen roses" thing; think outside the box---that's what your girlfriend is looking for.
Follow up with something like: "Come what may in our lives, I will carry this special moment frozen in time as my fondest memory of my life, to be remembered happily years from now, as if it happened moments ago...."
Anyway, you get the idea---good luck!!