I made friends with a genderqueer person, we'll call them Lex, about a year ago. It was the first friend I had ever had that preferred they/them pronouns, and let me tell you, it was difficult for me to get the hang of. When I first met them, I immediately assumed, based off their appearance, that they were female, and switching to they/them pronouns took some practice since I had never used them before. Any time I was speaking with someone and accidentally misgendered Lex, someone in the group would politely remind me of the correct pronoun. It helped being corrected in the moment, and over time, the gender neutral pronouns soon became second nature.

We live in a society that very much exists within a male/female paradigm. Though hopefully that will change as our understanding of gender changes, the vast majority of people have never encountered a situation where they had to use they/them pronouns. Maybe just politely reminding your T of your preferred pronouns every time she misgenders you could help. After a while, it will become ingrained in her that you use they/them pronouns, and it should become second nature. But, much like training a dog, it's helpful to correct her the moment it happens, rather than waiting for another day.