A sad and, alas, not such a rare situation: the girl begins to be jealous of her friend. Until she had a boyfriend, everything was fine. The girls shared their secrets, willingly chatted “about their own things, about women,” and went everywhere together. And when a friend admitted that she was in love, when she introduced her to her boyfriend, the second girl seemed to be replaced. She started to get jealous. Why does a friend have a boyfriend and she doesn't? Because of this, their relationship begins to deteriorate.
First of all, understand that you are simply being stupid. Think and analyze your behavior, try to understand that you need to fight jealousy. This is the first and absolutely necessary step.
You need to understand the question with maximum frankness: what is the reason for jealousy? What makes you frown with displeasure, suffer, seeing the happy faces of your friend and her boyfriend? Maybe envy is boiling in you: she is no better, why did she have a boyfriend, but you still do not. Or a wounded sense of ownership, they say, this is your friend, do not give her to anyone, her attention should belong only to you. Or maybe fear, and you think that if your girlfriend moves away from you, you will remain lonely, no one else wants to be friends with you.
If the cause of jealousy is jealousy, you need to understand that the sooner you get rid of it, the sooner you will meet your boyfriend. It is possible that a girlfriend is really no better in the sense that you put into this concept. Maybe her figure is not so slender, and her face is not so beautiful, and she dresses more modestly. But since the guy chose her, it means that he saw something in her that he really liked, which made your friend stand out from the background of many other girls.
Try to look at your friend through the eyes of her young man and understand: what is there in her that is not in you? Maybe you lack charm, kindness, modesty? Work on yourself, watch your behavior. This is much better and wiser than being jealous of a friend.
Well, if we are talking about the instinct of the owner, this is a really difficult case. Your girlfriend is not your property. And friendship does not and should not have anything to do with terry selfishness. Your jealousy harms you first of all, but your friend also delivers a lot of unpleasant minutes, and her patience is not unlimited. You should understand: either you decisively change your behavior, or you will soon be left without a girlfriend.
Maybe the reason is the fear of loneliness? This is typical of shy, withdrawn people. You should improve your self-esteem. Learn to get positive emotions, enjoy life. And then changes will not be long in coming.