I am sorry you are in turmoil, AJbear77. There are a couple of perspectives that may (or may not) help you.
One is to take what you said and apply it to a child telling his mother that he is worried she loves him less because she wants another child. We are all familiar with the idea of a parent sharing love across many children. So this may be an analogy that could work for you.
Another is to look at the dynamamics of your partner having a best friend (or several great friends). They will go out and do things on their own that takes away the time your partner has with you. People rarely have lives that include their partner 100% of the time. I think it works better to be grateful for the times we have together rather than resent the times we may have apart.
Also, there are benefits to your partner loving another. Sometimes sex drive increases and you may be getting more sex. You will also see them happy and compersion may work for you. You can relate to another person who loves your partner and can appreciate what they love about your partner. Someone who can help plan birthday parties or can be there for your partner when you are unable.
Someetimes poly people will choose monogamy for awhile (or as long as they desire). Some poly people have strict rules on what is and what is not allowed. I think the biggest thing to take away from polyamory is that it is about communicating our desires to the ones we love and realizing that love is not a finite resource (just time we have is finite).
If you need help trying to work through it, you can bounce ideas off of us. It sounds like the geanie is out of the lamp for you and you can't forget. So take time you need to reflect and let us know if we can provide any insights that may be helpful to you.
i wish you the best.