Well there is Sex, which is 90% Binary (Except for intersex, but that is a small outlier), and cannot be totally changed, and then there is Gender Identity, which is much more fluid and may change over time as people discover themselves more.
It's indeed very important to understand and internalize the difference between biological sex and biological gender, and the many different ways your body expresses different forms of this "male v. female" binary (or something outside of the binary), and how these many poorly-organized biological mechanisms can sometimes very strongly disagree. Biology is so very messy, and English's blending of all possible male-female concepts (and, of course, the stuff in-between) into two simple worlds helps not at all.
Sending love to all our queer friends out there, just trying to figure out what they even are. In general, don't worry too much about it, do what makes you feel right.