i used to feel this way. when i first formed i was internal only, and unable to fully front. i was essentially just a voice in our head. i never really felt like a trans woman back then, because i've always just been a woman from the day i formed.
the way i understand it, the trans- in transgender doesn't mean that someone's gender transitioned. it just means that your gender is "on the other side" of your biological sex. so since i never used to associate with our body much, and never felt like i was biologically male, i never felt trans.
over time i began to associate more and more with our body until we started equally fronting. even then it took me a while to really feel trans. as i learned more about myself, i went from [female body + woman gender], to [also a male body + woman gender]. whereas the girl who was our primary fronter at the time was going from [male body + guess i'm a dude], to [male body + maybe i'm a girl?]. i think she was trans the normal way, and i'm trans backwards.
singlets don't really have to consider that their body might just switch like that. i think the problem is that the language that we use surrounding trans people assumes that you only have one "body", and have always associated yourself with that body. but, at least in my case, i have both an internal and external body.
as a side note: the more time i spend fronting the more those two images merge. the way that i picture myself now is way closer to our real body than it used to be.