I know it feels awful to be subjected to this. However, if people don't realize they're making the wrong assumption, nothing will change. You do need to stand up for yourself, though, and gently correct people.
These days, if someone refers to me as she/her, I just say in a friendly and polite way, "It's 'him', actually, I'm a guy." Generally they apologize and we move on. I had someone in class refer to me as 'her' and I just spoke up and corrected him, which did make the class go awkwardly silent, but then he apologized to me.
I know it's daunting but you need to control your situation, you know?