"Living in your head, rent free!" I've heard this expression a few times and I've used it to help those around me, to understand that their resentment of someone or something is entirely of their own choosing. Think about this for a minute. Someone says something to get under your skin. When you let them get under your skin (meaning what they've said bothers your), you let them into your mind. The more you dwell and think about what they said, the more you let them live in your head rent free. At any moment, you may stop thinking about what they've said and kick them out of your head - the choice is entirely up to you! You are the landlord of your head!
Let this idea be your living principle and remind yourself constantly that "all that lies outside my [head] is nothing." Sticks and stones may break my bones, but words will never hurt me ... because only I allow myself which words can hurt me.