Let me first say, I get you. I am mono, but my man of 10 years is more fulfilled by more than one person in his life. It was very hard for me in the beginning as I wondered what I could do differently to be his everything. It took a lot of communication, sole searching and assessing my own needs to get to the place we are today. It's not easy, as if you look you will see that I have recently posted about my own jealousy/envy. From my perspective, although others may disagree, it never goes away but I did get better at handling it. Ultimately, you need to decide if you can be truly happy with someone who loves other men.
The other thing I have come to realize is that it's important to respect diversity of thought. While I understand why my partner needs other women in his life, it's not something I desire for myself. The perspectives you get on this forum will be mostly from the poly side. Think of it like the book Men are from Mars Women Are From Venus. The poly folks on the forum here are not attacking you, they are giving you a glimpse into their world. Use it as a gift.
Good luck as you navigate your way through. It is tough, but you will learn so much about yourself through the journey.