How about you write this on a card, feel free to change it to your liking. This is something I just came up with for you, hope it is sincere enough.
You are the one for me, it's easy to see.
I am the one for you, you know it's true.
As I see your face, and hear your laughter;
It's only you that I am after.
I miss you when we are apart;
please don't ever break my heart.
Be mine forever after!
Love Always, (your name)