The worst is not being able to trust other people much at all.
It's pretty logical, no?
Other people have managed to con you into believing you are not what you are, but something else instead. You find that out. You get mighty pissed, perhaps do a thing or two about it (like transition, maybe), and start really feeling the results. You're relatively safe from the forced assignments within your body and yourself - others may still mis- and/or ungender you, but at least you have a tendency of knowing yourself for what you are.
Here's the kick. When you realise that yes, most people have actively conned you (I'm not saying anything about consciously conning - it may well not be conscious, but the end result is still more or less the same) for, like, decades, it's not a happy moment. In fact, it may well feel like the ground opening under your feet. If other people can be so much in the wrong and so damn adamant about it and willing to spend so much time and energy defending their mistake, what else is there that they're wrong about? Suddenly, all bets are off.
This has, in my opinion, several consequences. One is that it's mighty hard to believe in yourself - in your own reasoning ability, in your own conclusions. No matter how well-founded your conclusions, you're still a human and still fallible. And since you've just had a glorious example of most of humanity failing big time, it doesn't bode well for you either, now does it?
Another is an active mistrust of others - if they managed to keep you from this central bit of information about yourself, what else are they hiding?
There's no easy answers to those. I don't know how to solve those problems - and they're my problems, too. How do you cobble together a trust broken from the very beginning? You've never had the experience of being able to trust - your very first relationships have been forced to fit into a mould that's simply wrong (I don't mean that for the cis it's completely right, but it is far less wrong as far as I can tell). There's no model of social stuff being right, only a broken model. I'm sorry this sounds so dejected, but the cissexist, cissupremacist world is a depressive place.
[Note that bodily issues don't enter into this much at all. Having a trans body is in itself relatively unproblematic - some aspects of your body might need medical attention, maybe surgery, maybe meds - and your body might not be that trans after all of those things - but that's just like having glasses, or corrective surgery, for defective vision. It's no big deal.]