Aren't we all!
This sort of story turns out, from my experience, to be pretty dang common, Thespian.
The root of the problem, if you were to ask my opinion, is that we live in a culture in which the poly- option isn't well known. Really, it's a matter of a need for social and cultural education. But as it stands at the moment, that education is proceeding slowly with a relatively few folks with courage and strength to challenge basic assumptions we all grew up not questioning. We've been taught by movies, magazines, love songs, parents, religious organizations..., that "Love means one plus one equals true love." And we've been taught that that's the ONLY valid and "true" kind of loving. And that's pure poppycock, and a lot of us now know this! Yes -- it's knowledge. It isn't mere opinion. It's a social fact. That is: Many, many of us can love two or more people at the same time and have that work out at least as well as the 1 + 1 approach. Often better.
But we have to acknowledge another social fact.: Most of us just don't "get it". We've been told by seemingly everyone, over many years, that love, the real thing, must be of the 1+1=true love variety. Most of us have bought into the myth--and it is a myth--that so much as showing interest in expanding that dynamic indicates un-love!
Well, for a long time, folks believed that slavery was perfectly acceptable, that women are less important or valuable than men, that homosexuals are immoral perverts, etc... etc.... And these people with these beliefs were very much emotionally invested in living out the myth they bought into. Racists will be VERY offended to have "niggers" compared with themselves as equals, then and now. So we still have a lot of work to do to expose the facts. And it is a fact that love, the real deal, need not be exclusive in the one+one way.
Fact: My partner and I love each other very much and have almost zero anxiety about letting the other love whom they will. Why? Because we are (a) secure in ourselves and (b) secure in the knowledge that we are truly loved by one another.
Why the "almost"? Because, while the anxiety is very, very small, it isn't non-existant, altogether. Because we're on a journey of healing from our conditioning within a culture that doesn't recognize the facts. We're human, after all.
A very strong love can always make this transition. The very risk itself can enliven a relationship. That is, the very honesty itself.