Anna, I had a doubt day not too long ago. I understand it sucks, it was like... do I look alright in these clothes? How come I can't seem to walk straight or properly? My voice keeps cracking and just being terrible.
There are definitely going to be off/doubt days for us, it even happens to cis gendered people. What I think can help is understanding if you're doubting yourself one day, understand it and try to make a small improvement the next day.
I think sometimes, the reality of having to present as one thing your entire life and then suddenly changing so much is bound to have some growing pains; and practice makes perfect.
Just today, I did doubt myself a little. I was always pretty happy with me voice, but sometimes when I listen to my youtube commentary i'm really unhappy. And I got this app on my phone, played around with it some today and basically showed my voice was getting worse, rather than better.
But. I used that opportunity to try even harder, and I think my voice actually(just a few hours later) has been the best I've ever had thus far.
S, like, amidst all my ramblings; I think use those low points to lift yourself up higher in the future, and utilize tools to help make it so. It could be a voice app, it could be like a dancing video game, it could be chat roulette(lol), just something where you can more easily measure your own progress in ways that just are hard to put to numbers some time.