I sympathize. I had a similar relationship in my early twenties, a MFF started organically but evolved into them as a couple and me as the "third." I tried to date others, to find my own "primary," but the truth was, I was not emotionally available to those others outside our threesome. I eventually left the relationship, and they got married.
There is no model in our society for how to do what you're doing, not even a proper name for what you are to them, and that can make it all seem impossible. Back when I was in my triad, early 1990's, we didn't even know what polyamory was, didn't have any resources or message boards, we were just winging it, and we were very young, and we made a mess of it. In later years, I went the monogamous route, got married, and that didn't turn out to be what the whole fantasy implies either....and now I'm here trying to make a semi-open marriage work...
I'd say try to take a good hard look at what IS apart from the whole fantasy of what you're "supposed" to have, then decide if what IS is good enough, or if in your heart you'll always want something more.