I chose monogamy every time I had a relationship ... It was great at first, then I'd get the familiar need for MORE, and either break up or cheat.
That said, perhaps you don't have so much of a drive to be poly as some folks? I certainly believe it is a spectrum.
Or, you could have turned your back on the poly relationship simply because it was too fraught with problems to work for you. Maybe you just needed that extra motivation of someone waiting for you to leave the couple? I don't see that as mono vs. poly. I see that as relationship that could work vs. relationship that isn't working.