My fellow player and I had started a 5e campaign with the plan that our two characters would be childhood friends that would eventually start a romance together. It worked well for a while, and I'll admit that I might've gotten too invested in the relationship as I planned out what eventually might happen between them, as I assumed that my partner was on the same page. Recently, however, she announced that she was going to play her character as polyamorous-- able to have romantic relationships with multiple people at once. Apparently she came to this decision after a flirtatious moment between her character and an NPC evolved into genuine romantic feelings. I have to admit that I'm very upset about this change, partially because she had never discussed it with me beforehand, and partially because my character has extreme trust issues and would not be at all happy with the idea that she would have to "share" him with someone else if they did end up together. I spoke to her about this and how it upset me, and she said that she apologized for not informing me of her decision beforehand but that she "had to play [her character] honestly." I should note that she's a very good friend of mine, nothing more or less, and my feelings about her decision are in no way influenced by our relationship outside of the campaign.
I feel as though she's backed me into a corner with only two unappealing options open to me-- have my character accept that her character will eventually seek love with other people, which I doubt she would be willing to do, or to just not have the romantic relationship come to fruition at all, which seems unfair considering all the planning we have done beforehand. I should also add that the DM in control of the NPC was also not given any indication that this would happen, and has no intention of roleplaying a romantic relationship, but the player in question has asserted that this would not change her decision to play her character as poly. This is my first campaign, though, which may have something to do with my reaction to this change in narrative-- though I've roleplayed in other mediums before, I'm not exactly used to the "yes, and..." aspect of improvising, especially when it comes on the heels of something we'd already planned for months.
I'm unsure where to go from here: how do I make this work?