Make your own rules. Rehearse with small, everyday things. For example, imagine that you are looking for a new apartment and an acquaintance emails you some pics of the place he’s trying to sell, with a note saying, “Non-negotiable price.” Well, you can’t afford it which makes you feel bad. As you have nothing to lose, reply with a response that empowers you. Say something like, “Not bad, but it doesn’t fit my criteria. Thanks for trying to help!” and see how much better you feel.