Don't assume the friendship is over. I know it hurts to know you can't confide in your friend like you thought you could, but you need to understand that this is a difficult thing for a lot of people to wrap their heads around. I went through a similar ordeal with my best friend when I deconverted from Christianity and became an atheist. He was convinced that I was "willfully rejecting God" and nothing I could say to him would convince him that, no, I honestly didn't believe this "God" idea was real any more. He couldn't fathom that someone could honestly not believe in God. I must have some hidden sinful motive. (in his view) We will never be able to relate on the level we once did, and that was a painful process for me to go through, but we are still on speaking terms. He is still a friend of mine, though he is no longer my closest friend.
Also, it is entirely possible your friend might eventually be able to understand polyamory. It's not guaranteed, but some of my religious friends I knew from back then confided in me that they knew how I felt. That they had doubted/questioned the religion too. I think the principle carries over to this context.
If you want to continue conversations with her on this, I would highly recommend using the Socratic method with her. If she feels like a person can't commit to more than one person or love more than one person at a time, ask her why she feels that way. Try to uncover the reasons behind her strong feelings, without judging (easier said than done, I know!) For example, try asking her if she feels it is possible for a parent to love more than one child. If yes, then ask her if she thinks having more than one child will mean the parent will love the first child less. If she says one person is supposed to be your one true love and give you everything you've ever wanted, ask her if she's ever seen any married couple that always gave each other everything they ever wanted. If she talks about finding someone to spend your life with, ask her why she thinks a group of say, three people, couldn't decide they all wanted to spend their lives together. Couldn't they all be faithful to each other? Why is being faithful to one incompatible with being faithful to all? Ask her if she thinks being in love with one person means you should act cold and unloving toward everyone else. If not, why not? The trick is to uncover the reasons why she feels the way she does without it feeling like her own beliefs are under attack. If you do that she will just get defensive (like she did in the conversation you posted) and your conversations won't make any progress. I wish you the best of luck! I know how hard something like this can be.