I don't have this fear, but that's because I observed myself in life as if I was watching another person, at the same time as I've been living life. I got to know exactly what my habits and tendencies are outside of my conscious thinking, how I end up reacting to stuff and the usual patterns of my own behavior.
By now I think I've had long enough to see that some things are never going to change - they are permanent problems. Things like genital dysphoria, sexual dysphoria, avoidant habits... I've tried different ways of climbing over them and come back to the same conclusion. They're ingrained and beyond my control and I definitely, absolutely, have a complex in there somewhere relating to gender of the body vs the mind. The only possible chance of fixing them is to transition. I'm aware too that transition also may not fix them, or might only go some of the way to helping. But I've had enough time to be ready for that.
Just like everyone else I had moments when I wondered if my mind was lying to me.
But then even if it was, my life as a female was completely miserable. I'd rather take the chance and see.
And well, if it turns out not to be what you hoped what's the worst thing? You'd de-transition and go back? People have done that and turned out ok too.
If it's not obvious from my posts on here I also have a real nasty fear of being wrong. But there's enough cumulative evidence stacked in favor of transition to risk it now.