Summarize this article:

When someone is feeling jealous, you want to let them express themselves. Even if the jealousy is baseless, do not tell your girlfriend she's being irrational. Allow her to tell you how she feels and listen in a non-judgmental manner.  Remember, your girlfriend may very well know her jealousy is irrational; therefore, responding with something like, "Not this again," or, "You know you have nothing to worry about" is not a good means of deflecting jealousy. Instead, allow her to discuss her insecurities frankly. Your girlfriend may be more willing to move past her insecurities if you allow her to talk to them through rather than cutting her off. You can even encourage her to share, by asking her something like, "Can you tell me what you're feeling insecure about right now?" You do not want to tell your girlfriend not to feel what she's feeling. If your girlfriend is being jealous, your natural inclination may be to get defensive; however, this will not help diffuse the situation.  Instead of responding defensively, show your girlfriend she is heard. Do not, for example, say something like, "You know Melissa is just a friend." Show your girlfriend you're hearing what she's saying, even if you don't agree with it. For example, "I understand you feel a little threatened by my close friendship with Melissa." Oftentimes, another person's jealousy can cause you to retreat. If your girlfriend is being jealous, you may get frustrated and feel she's being unfair; however, keep things open. Respond to what she's saying, even if it's frustrating. If you shut out someone who's feeling jealous, this will make the problem worse. Resist the urge to retreat or ignore your girlfriend when she's being jealous. Even if her accusations are unreasonable, remind yourself you need to let her speak. If necessary, take a few deep breaths. "I"-statements are statements made in a way that deflects blame. You focus on your own feelings about a situation rather than making an objective judgment. After your girlfriend has expressed herself, calmly address the situation using "I"-statements.  "I"-statements have three parts. They begin with "I feel..." after which you immediately state your feelings. From there, you explain the behavior that led to that feeling. Lastly, you say why you feel the way you do. For example, "I feel frustrated when you interrogate me about spending time with Melissa because I want to be able to see my female friends on occasion." Oftentimes, jealous people have underlying insecurities. Instead of responding to jealousy with anger, reassure your girlfriend how much you value her. Remind her that you love and care. For example, say something like, "I get that it must be weird for you that I'm friends with my ex but she's my ex for a reason. I really love you, and really cherish our relationship."

Summary:
Allow her to discuss her insecurities. Validate her feelings. Work on being responsive. Use "I"-statements. Provide reassurance.