Yeah but look, you can believe that you are female when you are male and that you are male when you are female, but mentally considering yourself of the opposite gender doesn't make you of the opposite gender.
It's like wishing for Santa to be true, no matter how hard you wish, Santa will never be real. Just like that no matter how hard you wish, you won't become female/male just by thinking about it.
And to put it bluntly, the only thing that matters towards what gender you are is if you have a snake or a vagina, if you have a snake you get to be called a male, if you have a vagina you get to be called a female. Nothing can alter the fact that you remain who you were born as, there are some specific things that you aren't in control of and possibly never will be, it's much better to accept the fact that you can't change who you were born as rather than throwing a temper tantrum on the streets about the fact that you got the "unlucky" draw and were born as the opposite gender of what you wanted.