Hugs, Luna Star.
I suspect this, like most of our collective trans-related neuroses and fears, is all in your head. Are you seeing signs from others that they're not accepting you as a girl? (And if you are seeing such signs, are you actually seeing them or are you merely imagining them?) Chances are, very few people care about this other than you!
I started out with the same worries. It's difficult. Comfort comes with repetition, like being the new kid in school. Remember those feelings? As if you'll never fit in, that you don't understand the rules or where things are? And everyone around you seems so comfortable and knows all these things you don't?
But you remember how you eventually figured it out? And looking back, nobody really cared that you were new?
Not sure that helps, but I wanted to throw it out there anyway. Even at this stage in my transition, I have days where I feel like I'm faking being female. I think many of us do. My strategy for coping with this is to forget about trying to be female (which I personally think I'll never reach, no matter how much time, money, surgery, and therapy I throw at the problem), and to focus on the thing I do have control over, which is being a great "me".
And that "me" is generally female, passable most of the time, and happy that I'm moving gradually further away from the guy I once was. A work in progress.
But to get back to the original point, perhaps focus on a less distant target. (And as a brief aside, most of us can get to a decent level of femininity if we put in the time and effort. You will get there, even if it might seem miles away right now.) Distant goals are often hard to achieve. How about focusing on reaching something smaller that will give you renewed purpose and a sense of satisfaction when you achieve it? Not sure what stage in your transition you're at, but take pride in each of the baby steps you make.
Also, maybe spend some time thinking about what being female means to you? Are you setting your sights on an impossible goal (for me, this would be becoming a young, pretty girl - I'm not young, nor pretty!) Women come in all shapes and sizes, and they're all equally female. Surely there's a niche somewhere in that spectrum where you'd fit rather neatly?
Not sure if any of that helps, so I'll pass on another hug. Sorry you're feeling rotten.