Hi and welcome
No doubt some of the more experienced members will jump in here and give you some links to writing on jealousy. It's one of the recurring biggies for polyamory.
I'm fairly new to all this myself and still working through the jealousy. My partner's OSO lives quite far away and so my jealous outbursts generally only come up when he is visiting her and that hasn't been for a while. I know I'm not over it though, just working through.
Try to look underneath the jealousy to find out what other feelings snd issues are at work. For me there is
-loneliness (after being married for most of my adult life I'm not used to being alone, I accept that learning to be alone or with other people and happy is a healthy thing)
-Fairness. I don't get much quality time with my partner due to his children and yet I put a lot of work into helping create a good environment for them to be in. He has acknowledged thatflipping off too much to be with his OSO isn't fair.
-Assertiveness and trust. I have learned to trust that I can be assertive and reach a compromise with my partner and he will still love me and can be happy. Previously I have given too much away and been miserable as a result.
So while it hurts like hell, jealously can be an opportunity for growth. While I'm not looking forward to Z going away to be with his OSO I will be quite interested to see how far I've come in terms of jealousy.
Hope this helps