I was with this really amazing girl - the love of my life, and she and I are both poly. But her main wasn't. Her main said that she wasn't comfortable anymore with the fact her girlfriend was dating other people. I thought that maybe I would be chosen. That she could love me more. And we could live our lifestyle together. But no. They're getting married next month. I still love her so much. I want so much to be a part of her life. I could even deal with her partner not liking me. I just need her.
It's okay to grieve the end of this relationship, but there's nothing else to be done - I am sorry to be the one to tell you that. Sometimes this happens, not only to polyamorous people, but to monogamous people as well. A partner who you love chooses their work, or the freedom of singlehood, or another partner, or something else, over their relationship with you.
And it hurts. It really does. But you will survive. It feels like you 'need' her, but you don't 'need' her like you need air in your lungs or food in your belly, even if the loss of her does feel like drowning or starving. You will get through this. It's okay to mourn, to be angry, to feel hurt. Breakups suck. Loss is painful.
Do whatever you need to do to feel okay: Consider seeing a therapist who specializes in grief at the end of relationships. Eat a bunch of ice cream. Join an online dating site. Go for a long bike ride. Have some drinks with your friends. Block your ex on all social media so you don't see any wedding nonsense. Pet a dog. Go camping. Write a letter to your ex and then burn it. You'll get through this, I promise.