Really, you're better off just letting it go and counting it as an expensive lesson learned. You never, ever LOAN money to friends and relatives; you GIVE it and expect to never see it again. If you happen to get repaid, it's a pleasant surprise.
This festering resentment is not going to do you any good; it will eventually explode and you'll end up with far fewer people around that will want to help you out, should you ever far on hard times. Let it go and remember to never loan out what you can't afford or aren't willing to completely surrender.