I think you may be harder on yourself than you need to be. People get broken hearts all the time in regular monogamous relationships. So it is natural to also get that behavior in poly relationships as wel. It sucks, but it doesn't mean that you did anything wrong.
All you can do is learn from it and try not to repeat any similar mistakes in your next relationship. It is a different form of relationship that many people are not use to, so naturally people want to criticize it.