I was gonna make a thread similar to this. I may still but ahh. I'm glad I found this one.
I do not have experience with poly relationships. However, my partner does and that's his preference for reasons I don't really understand, but whatever. We're thinking off breaking off our monogamy and experimenting with an open or poly relationship. We haven't actually done anything about it yet, but we are discussing it heavily. I experience the same feelings of jealousy, as well as anxiety about whether or not he'll leave me for someone else. It's terrifying. But we plan on getting married and neither of us feel comfortable with the idea of being bound to one person, sexually or emotionally, forever. So I want to face that fear.
The first thing that we've done, which I think should apply to all open or poly relationships: put your foot down and declare that you NEED to be the priority. If your partner is not officially in a relationship with the other person, or is just sleeping with them, YOU need to have priority over them. If they are unwilling to turn their attention back to you, it is more likely that they prefer the other person and are straying. Of course, you need to talk to your partner first and tell them this. Express that you need attention. When my partner said that he wanted another sexual partner, I flat-out told him that I have to be more important than the other person. If I am upset, if I need him there, if I want to hang out with him, if I want to go on a date, I come first. He doesn't necessarily need to cancel plans with his other partner to be with me, but he needs to make sure that I am okay with him spending time with this other person instead of me.
If you and your partner are both comfortable with this, meet the other person. Maybe even befriend them. If you like this person, if you get to know this person, if you can feel involved, you may feel better about your partner seeing them. If you guys like each other, the three of you could even hang out together. You'll stop seeing them as 'this other person who's sleeping with my gf' and more like 'someone who is also able to make my partner happy'. Which can also encourage jealousy if you overthink it, but if genuinely care about your partner's happiness, it should make you feel comforted.
Whether or not this works depends on your understanding, as well as your partners understanding and willingness to tend to your needs. If you're still experiencing jealously weeks or even months from now, maybe you should look into therapy, soul-searching, or a break. A lot of times jealousy comes from one of two things: fear or low self-esteem. If you fear losing your partner, discuss this with them. Seek reassurance. If that doesn't work, try to build things and relationships that will be there for you if she leaves. It may seem like the end of the world, but it doesn't have to be. Especially if you have a good career, a good social life, pets, and a healthy home. If it's a self-esteem issue, write a list of what you want to work on. Set realistic goals. But remember that your self-esteem doesn't stop at the skin. You could change yourself cosmetically as much as you'd like, but if your lack of self-esteem comes from trauma, a confidence issue, or personal/mental things you don't like about yourself, you need to pay attention to that as well.
Most importantly, this is not something you can force. Like others have stated, just because you want it, doesn't mean that it's right for you. If you try or have tried everything and you are still hurting, you'd just be beating a dead horse by seeking out other 'methods' to cope with this. Sometimes there is a point where you have to walk away from a relationship for the sake of your mental health.
