Your biology dictates what you are, what you will become, and what experiences you will have. Gender, sexuality, puberty and all the icky-ness that goes with it, are not yours to command. They just happen to you, and you grow into the socially constructed aspects of it, as you encounter the world in the guise of a man or a woman, and as the world responds to you as such.
Trans men and women resist this process. Admittedly, they are driven by thoughts and feelings that may also be biologically determined, or they may be predisposed to developing certain sorts of thought patterns. The brain is an organ of biological processes, after all.
However, the body (and brain) will continue to develop according to the plan laid down by its genes, whatever you may feel you want. If you have a Y chromosome you are fated to be biologically male, whether you like it or not. If you don’t, you are undeniably female. Nature seems to be under no obligation to make you happy or to resolve your inner conflicts. Inner conflicts are the condition of being human.
Of course, people with gender identity issues feel deeply distressed by this. They should be given immense amounts of sympathy and support. But this does not mean that they are conceiving of their problems with clarity, or that their solutions are the correct ones. Having anorexia, and then therapy, has taught me that urges, assumptions and beliefs I thought were fundamental truths, beyond rationality, unassailable to reason, can be tissues of falsehood. Acknowledging this can be deeply disturbing; it can seem completely counter-intuitive, yet over time you learn to accept that your rigidly held beliefs had trapped you in a limiting box.
On the other hand, anorexia also showed me that you could accept the untruth and irrationality of a position or type of behaviour, and still compulsively pursue it.