Explain your dilemma. Be clear and open with your girlfriend about the fact that hanging out with her ex-boyfriend is making you jealous. It is essential to differentiate normal jealousy from delusional jealousy, according to the book "Romantic Jealousy: Understanding and Conquering the Shadow of Love," by clinical psychologist Ayala M. Pines. Delusional jealousy occurs when there is no actual threat to a relationship, but normal jealousy is based in a real threat. Explain to your girlfriend that the relationship with her ex-boyfriend is enough to make you feel envious -- for a good reason.

Listen to her. You may not be interested in listening to what your girlfriend has to say, but it might help you see the situation from her perspective. While you do not have to agree with her, listening will help you understand why she is continuing to hang out with her ex-boyfriend. It will also show her that you are interested in her feelings, even though you may not accept her actions. You may find yourself feeling less threatened than before. This will help clear your mind as the two of you figure out how to solve this challenge in your relationship.

Establish boundaries in your relationship. It is a good idea to figure out where both of you stand on this issue. You might suggest an all or nothing approach if you are not comfortable with her spending any time with her ex-boyfriend. She may not like this idea, but if she values your relationship, she will prioritize it over her friendship with the ex. You might be okay with her spending time with him, as long as it is in a group setting and you are around. This way, she will see her friend while keeping you involved. Figuring out what both of you want in your relationship is a good way to deal with you and your girlfriend's conflicting desires.

Offer an ultimatum. Old relationships are delicate topics between lovers, and you are in a vulnerable situation. If your girlfriend refuses to listen to your perspective and cannot see why you are uncomfortable with her behavior, it may be time to offer an ultimatum: either she agrees to establish boundaries, or the two of you will break up.While being extremely jealous is unhealthy, it is natural and expected that you would not want your girlfriend spending time with her ex. It may seem extreme, but there is nothing extreme about your desire to be respected, heard and prioritized by your girlfriend.