It's not being the other gender that's exhausting, it's all the work of getting others to accept me and me to accept myself. Staying cis used to be a comforting idea compared to what I'd have to do to actually live out life as a trans woman but now that I'm actually on that path the idea of going back is pretty horrible.

One mental exercise that may be helpful for you is the quintessential button test. Imagine pressing a button and you no longer had to try to be a woman. Regardless of what you wore or how good you looked on a given day, everyone would unquestioningly accept you as a woman. More importantly, you could fully accept yourself as a woman and not have to think about it all the time. Whether or not you would press the button is an important answer, but I think it would also help to think about whether that idea offers a sense of relief in the way that thinking you might be cis after all does.

Hope this helps in some way!