I don't give advice/feedback very often but I sympathize with your situation amd felt like I could at least offer emotional support.
A framework that is helpful to both myself and one of my secondaries is thinking about our relationships as friendships that may or may not include a sexual or romantic component. Thinking about my primary partner as my super crazy best friend and my secondary relationships as very deep friendships that sometime include sex helps me keep perspective on everything. It's also really helpful for me in not getting overwhelmed by the relationships that my primary partner develops.
I think, too, it's worth considering the fact that we are conditioned to allow ourselves to love multiple friends and family members. What is it about for you that is so challenging about your partner having multiple romantic loves? I realize that's a tough question
I'm still in process of managing that myself. Hang in there