Whether it's warranted or not, your ex's best friend may not be able to trust you if she thinks you have wronged her comrade. If you want to stay friends with this person, or you simply want her to stop trashing your name, you stand a chance of repairing the situation: Keep your cool as you apologize, show you've changed for the better, and make amends where needed.

Deal With the Anger

Whereas responding to someone's anger with your own will keep the cycle going, responding calmly can defuse explosive emotions. If word reaches you that your ex's best friend has spread hateful rumors about you, try asking her why she has done so. Regardless of the answer, try to make your own response level-headed, even empathetic. Staving off the urge to fight fire with fire lets her know that you are not a threat and may therefore deflate her anger, according to Margarita Tartakovsky on Psych Central.

Promise to Change

If you have done something wrong, the quickest road to restoring trust involves addressing what you will do differently in the future and reminding her that people can change. In the study "How Implicit Beliefs Influence Trust," published in 2010 in "Psychological Science," Maurice Scheitzer of the University of Pennsylvania's Wharton School and fellow researchers studied the willingness of subjects to hand over money -- for potential profit -- when the would-be recipient had refused to share the gains once before, but promised to do differently in the future. Those subjects exposed in advance to a message that people can change were more likely to trust again. Whether to trust you again is ultimately in the friend's court, and depends on her personal beliefs, but you can influence her to trust you.

Right the Wrongs

If you contributed to the dissolution of your relationship by cheating, acting insensitively, neglecting your girlfriend or being dishonest, trying to make amends will not only impress your ex's best friend but also clear away some of the rational guilt you're most likely experiencing, according to Psych Central CEO and founder John M. Grohol in "5 Tips for Dealing with Guilt." While making restitution can be complicated when it involves the intangible, such as trust and hurt feelings, you can go about it by sincerely apologizing, emphasizing the lesson you have learned and asking whether there's anything you can do to make it up to your ex-girlfriend.

Give It Time

If she has reason to mistrust you, your ex's best friend will need evidence that you have changed for the better. In this case, your best bet is to treat those around you, including her, with fairness, tell the truth and offer a helping hand when you're able. Even if she does not personally observe your honest interactions and acts of generosity, word may make its way to her if you are consistent in your dealings with others. If her anger toward you is rooted exclusively in loyalty to her friend, rather than more objective disapproval, time will also help; she will need the opportunity to cool down and see you as an individual apart from your recently terminated relationship.