The most romantic way to say "I miss you" is to express your feelings from the heart and use poetic, heartfelt language. Here's a romantic way to say it:
"My heart aches in your absence, and every moment without you feels like an eternity. Your absence leaves a void that only your presence can fill. I long to be wrapped in your arms, to feel your touch, and to see the sparkle in your eyes. You are the missing piece that completes me, and I yearn for the day when we can be together again. Until then, know that you are always on my mind and forever in my heart."
Feel free to personalize the message with your own emotions and experiences to make it even more special and meaningful. Remember, sincerity and genuine feelings are what make romantic expressions truly powerful.