Hey there, attempting to sneak in a review for Team Tortoise before my counter restarts!
I would normally start with nit-picks but you only have one which is a repeat of "to be happy" in the first line of the second paragraph. So I'll go to overall stuff.
The thing here that I like is the idea of someone being in love with both people in the relationship. I can see to this leading to some really interesting problems and interpersonal dynamics. However, I think I could do with a bit more from this prologue. What you have here is sort of more like a blurb if that makes sense? Like, you're telling me what the concept of the story is. But what I don't have is an ability to invest in the characters. You're telling me a bit about what they're like, but I don't get to see any of the actions they do or the things they say or the interactions they have with the narrator. This means I have to take the narrator's word for it that this character I've never heard of before is something really, really special. I know it's a prologue so it doesn't need to be as long as a chapter, but I think you could benefit from this being set in some sort of scene that doesn't have immediate plot relevance, like just an average day where you can show what life is like. Alternatively, you could keep it this way and just include more details that are particular to these characters - maybe mannerisms or catchphrases - something specific to make these people feel unique.
I also don't think that someone is necessarily expected to be happy for their ex when their ex gets into a new relationship. That's understandably painful, especially if that person was left for that new relationship. I'm actually not sure I understand why the narrator is still in their lives. They could have agreed to be friends, but that's not guaranteed to happen when a couple splits up so you'd have to establish why that was the case I think. I mean I can understand wanting to stay around people you're in love with, but I can also imagine being rather annoyed at being left for someone else.
One more thing! I adore you for using gender neutral pronouns for one of your characters (non-binary gender?) and for having a narrator who seems to be bisexual/biromantic. Happy pride month! *waves flag*
Hope this helps,
Biscuits 
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