How did you meet each other? Before you met each other, did you not have other people you were seeing that it didn't work out with? Why would it be any different trying to find a third partner? I don't think you should be so discouraged just because the first person you dated as a unit didn't work out.
Also, there will be no easy answer for you because people are still people and the vast majority of people do not make ideal partners for the vast majority of people, so my advice is to really be appreciative of the one relationship you already have and not get too hung up on "finding" another partner.
My attitude toward this kind of "seeking people" thing is not very popular around here. I prefer to engage in activities and socializing based on what I want to do, not based on whether it might yield potential partners. I joined roller derby because I want to skate - however there are lots of potential social connections to be made, but only now, after 2 years, am I beginning to REALLY connect with some of the other people. Same with recreational nudism - I go to the clothing optional campgrounds because I like to be naked and I hate wet bathing suits - however there are lots of potential social connections to be made. Although I am interested in medieval stuff, I do not have time to join the SCA - but if I did, I'd join because i want to do medieval stuff, not because there might be polyamorous people there.
If you don't like to go out or whatever, then I guess you're just going to have to keep shopping the internet and try to make that work for you.