Let's rephrase it:
"How empty of me to be so full of you." --> "There's nothing inside of me but you."
The author is using this quote to point out the importance of maintaining your independence in a relationship. A person shouldn't dominate or be dominated by his or her significant other. General Lesson: Don't give too much of yourself that you lose sight of who you are as an individual. Remember to also consider your own needs and happiness.