Your life is long, there will be plenty of times when you will be separated. Quite a lot of times you will be missing her, and she will be missing your. However, if you will start repeating her daily I missed you, you will not get far away. Some girls might enjoy that, however for others it might be annoying. Instead of saying I miss you, next time say I miss your smile.
Wow?! You have just expressed yourself in the same way but with different words. And if you are on the phone, or talking via chat program, you have just made her smile again.