Fear or resentment that something you have will be taken away - like a partner, quality time you share with that partner, position in someone's life, privileges, etc. Jealousy is a natural emotion. Being non-monogamous does not void you of the emotion. It also doesn't mean you are a terrible partner for having those feelings. The feelings of jealousy are not the issue typically - what you do with those feelings is when they can become a problem.
You can't trust your partner when they are with someone else.
You fear losing your position in their life.
Your fear or resent your partner for sharing something with them that you won't have access to or had access to you
You attempt to control your partner(s) behavior and what they do with others.
You may try (or think) about sabotaging their plans with others. - This includes making sure they are too busy to do anything else.
You tend to make rules versus boundaries. - "You can't kiss other people.", "You can't take trips with your partners."
Jealousy =/= Love
Jealousy is not a sign of love or a measurement of how much you love your partner(s). It reflects how you perceive your partner(s) and your insecurities. Sometimes we see our partner(s) and the things they do with us as possessions. One big thing we have to unlearn in relationships is how society packages toxic jealousy as something that represents love and devotion. Popular culture promotes the harmful acts stemming from jealousy as symbolic of deep love and desire. The jealous partner sabotages their partner's opportunities, and it's painted as a cute lover's quarrel. Someone deliberately does things to set their partner off (get them jealous) because they feel left out or not desired, which is seen as harmless and not manipulative. We should actively unlearn these things in monogamous AND polyamorous relationships, and they do not disappear in polyamory.
A lot of jealous feelings can be processed alone with self-reflection and personal accountability. Is this a trauma from a previous relationship/event causing you to feel this way? Do you feel like you own your partner? Do you find some activities between your partner(s) sacred? Have you expressed this to your partner(s)? Are you insecure about your importance to your partner? Are you worried about being displaced?
Communicate your feelings of jealousy if you are unable to process them alone. Sometimes your feelings of jealousy may be observations of unfair situations. Your jealousy may be a reaction to something that needs to be addressed. It is a vulnerable position to be transparent that you are having feelings of jealousy. It may not be comfortable to talk about initially, but there are ways to open up the conversation with your lover(s):
"I have been feeling jealous of your relationship with ------- lately, and I want to spend more time with you as well."
"I don't like that ------- shared that experience with you, and I felt like that was a special thing only between us."
"I am scared that I may lose importance to you. It feels like your relationship with our partner is stronger than with me..."
"I have been feeling weird about your other relationships and need some help processing that."
Additionally, when you communicate, it brings clarity to the situation at hand. There is a chance you misunderstood what was going on. Perhaps your partner is guilty of overstepping boundaries or has been giving you and your relationship less attention.
Jealousy doesn't disappear overnight, and you can't ignore it. Unpacking your baggage and communicating concerns and feelings of insecurity to your partner(s) is necessary to process your jealousy. IT IS OKAY TO FEEL JEALOUS. Punishing and controlling your partner(s) for your feelings of inadequacy or seeing them as an object that belongs to you isn't okay. Be mindful of your jealousy and how you handle it - as a tool for a better relationship or an aid for abuse.