I spent my childhood being bullied and teased. I learned to develop a border that I do not allow anyone to cross. It makes me appear strong or unfeeling at times but I would rather people fear me rather than use me as a door mat or garbage bag. I have learned to protect myself. Others ask how I don't let myself be pushed around. I tell them my secrets and leave them to it. Success is entirely up to them. I've seen two people succeed. The trick to it is to never back down. No matter what way they come at you, readjust and build a new wall as quickly as possible. Don't get me wrong, it is possible to remain bright and friendly, just erect a barrier that you refuse to allow certain things to pass through. Feel like you're being used? Just say no. Don't give explanations or any discussions about it. Just leave it at that. No. Anything else and the person will keep pressuring you believing they can make you change your mind. This will leave both sides angry and frustrated. Unfortunately, this will also leave you open to the same topic again.
You don't have to become cold and heartless. That is not the way to live. You will eventually become miserable, mean, and crotchety. Just make up your mind about what you will or won't allow and stick to that. If you must say something DO NOT dissect and overly explain your decision. Keep it to the point and as brief as possible. This is used only when speaking to an authority figure such as your boss.
Well, that's my advice. Good luck.