Remember, that she does not have to go away in order for you to say her I missed you. These words carry a huge power within them… if you miss someone, this means you care about them, you think about them, you feel good when you are with them. All of these things are expressed when you say I missed you.
Imagine waiting for your girlfriend at your doorstep in the evening. When she is finally home, hug her and tell her how much you’ve missed her. Even so, you were together in the morning.