Hmm....Well, my first instinct is to tell you not to apologize, but that is bad advice. It's good to be able to admit when you were wrong and neither party can heal until the guilty-for lack of a better term-owns up to his/her actions. I recommend acknowledging what you did might have hurt this person and simply apologizing. Show that you care how your actions affected this person. "I know that when I did this, it upset you and I apologize. I was in a foul mood and took my frustrations out on you." Would be an example.