I'm a bisexual woman-I know what you mean about the confusion.
I don't have a girlfiriend-haven't in YEARS.... fear I remain in love with the unattainable ex-girlfriend.
it's hard to explain to my husband and boyfriend that there is something missing-but it's not them or their fault.
I have found (on accident) that by immersing myself in the LGBT communities, I have found other poly's who are out as LGBT, but NOT out as poly. That may be an option for you to explore.
Also-read read read as the last poster said. There's such a wealth of information on this topic and you will gain so much insight into how to get what you want if you figure out exactly what it is that you need.
And-keep your mind open, you may find one and end up being "the couple" that attracts the other part of what you desire.