It sounds like the person you were with would have been better off in an open relationship with someone.rather than labelling it as polyamory or want to pursue polyamory?
I've not been in a ployamerous relationship myself but I'd imagine the hardest part is the time and effort needed to maintain your relationship with each partner?
I could see 2 partners being doable but hard work, but once you go beyond that, then it must get very difficult? Especially if you don't all live together as juggling full time work around making the time and space to maintain very close personal relationships must be very hard.
And my mind boggles when you get to pplyamorpus "networks" where 2 partners may have relationships with other people rather than a shared 3rd partner. I think it would take a lot of honesty and maturity to make that work long term. I don't think I'd be capable of that.