I don´t know if mental illness and/or anxiety are often accompanied by irrational fears, but I have trouble with these from time to time. It´s nothing like the terrors that used to haunt me as a kid (the monsters under the bed), but sometimes it will still be comparable. For instance, I have a fear of deep water (especially when I can´t see what´s in it), and a fear of heights.
Depending on the type of fear it will induce me to act differently. My fear of deep water stops me from swimming all places I would like to, and from going in deeper. My fear of heights does not stop me from my love of flying, but there have been several times where I was clutching the yoke with cold sweat on my forehead and trying not to think about the height.
Gender dysphoria also brings along quite a deal of fear, for me. This is the fear of things staying the way they are. To an extent, it might be totally natural – specially for those of us who have conformed to their birth sex for a long time – but when fear starts to induce panic and other emotions, it starts to be unhealthy.
The fear of not changing physically is something I´m struggling with, even though it´s mostly irrational. The underlying emotion is not so much whether I´ll be muscular but whether I´ll be free to express myself – the kind of freedom that I´ve felt was lacking from my life for a very, very long time. Whether I´ll be perceived the way I am.
I guess there is not much you can do about fear rather than realize that fear is not based on rational thought but rather on bad experiences. It looks at the world from a negative perspective, assuming that the experiences are going to repeat. And then, when you start to believe the fear, you feel threatened and react to that. A vicious circle.
To those of you who are afraid and to myself I´ll say don´t worry. That deep water is not going to bite you, and that high altitude is only showing you things that you have never seen before. Things don´t stay stagnant in life when you let go of them, so it´s better that we let go of past experiences and make place for something new.