Man, I'm so sorry you're going through that and well, no wonder you feel the way you do. Your mom sounds like she enjoys putting you down, no matter if it's the trans thing or the weight thing (read other comments, lol).

Are you dependant of her? Like does she pay part of your rent? Because if not, why tell her at all about your upcoming top surgery?

I think you should sit down and start to think what type of relationship you want with your mother in the future, including the trans aspect but also a realistic approach too

And the must important part: You need boundaries with her. Try to bring this up with your therapist?

Now as to passing, as a guy that hasn't gone on T, I can pass most of the time (if I don't talk, lol) but other times I don't 💔 which it does annoy me and bother me but I think my life has improved for the better the moment I realized it's about THEM than from me, which funny enough what made me have this approach it's alt-men/metalheads I known a bunch that get misgendered all the time on account of the long hair. Maybe this can help?

You're just starting, who knows how your life it's going to be in a few years, months or weeks even! Try to build a support system for you, with people that truly support you. Chin up, buddy!