Oh sweetie, what a conundrum.
As one of Dena's difficult cases
firmly in the non binary camp it can be difficult to grasp - it's like the certainty that we are lead to expect of our basic understanding of ourselves isn't there. For some it can come and go, others it's absent, and for more still it's only part way, or a blend. To recognise it takes introspection, and time. Ideally with a supportive therapist along the way.
If you are feeling mostly feminine, then transition may be right for you - we have members who are gender fluid, and transitioned to be able to be comfortable for the majority of time with their internal sense of gender. Where it gets tricky is when our sense of gender matches neither pattern that is common - neutrois, or when we have no sense of gender (agender).
In terms of biology, my belief is that it is an incomplete process, or sets of processes - that we have brains that are effectively intersex, without the external morphological differences that the better know and understood intersex conditions produce. To find a source or reason can feel like searching for needles in a haystack. Who knows.
The fact that we understand a little more about the binary brain formations and implications on gender, and that it is an active area of ongoing research, show that these biological foundations that are poorly understood. Prenatal exposure to DES, by your grandmother could be enough, o possibly even your great grandmother - as they are finding that the diet of tulip bulbs that were commonly eaten in the Netherlands in ww2 have had effects that have been propagated down generations without direct alteration of the DNA, it's a complex subject, where we still know so very little on how our environment actually influences development especially in utero.
I'm sure that @Devlyn will be along soon - she'll tell her own story without a doubt.
The only certainty I can give is a (hug) and support that you're not alone.
Rowan