I had doubts ALL through transition. And again, they weren't based on any sort of concrete reality, they were just based on the fear of "what if I'm not really trans?" or "what if I'm not really female, what if I'm just a super-effeminate twinky guy?"
The thing that I always asked myself, and I think you could benefit from it too, is constantly asking myself "do you want to go back?" I imagined being back on testosterone, having body hair again, dealing with MPB again, the male sex-drive again, and being socially treated as male again. And as long as I was still revolting at that notion, and the answer was no, I couldn't go back to living like that, and therefore as long as there was nothing concrete making me want to change my mind and go back to being male, I reasoned that there was nothing to worry about. I also had a lot of self-critical bouts along the line of "If I could be a cis female I would do it in a heartbeat, I wouldn't be having these doubts, but I'm not sure if I'm strong enough to be a trans female even though I probably identify as female." So really the doubts were coming in because I was wishing that I could be MORE feminine, not less, and I realized that going back would solve nothing, it would just get me even further away from that ideal self that I wish I was.
My therapist asked me, after a weeping session where I was crying about not being "female enough," "Do you regret starting transition?" And I answered no, I could never go back off of hormones. And her response was "then focus on that. Quit focusing on whether you live up to some ideal standard of femininity or not."
I'm not sure if our reasons are similar, but I imagine that your mind is just doing the same thing that mine was doing... overthinking it. Just relax. Unless you actually have concrete regrets, a legitimate desire to go back, then there's nothing to worry about. You'll know if you've ever gone too far.
(Side note: I'm still having doubts. Mainly because wearing makeup every day and planning outfits every day and trying to live up to female standards of beauty every day is a royal pain in the a**. But at the same time, I still don't want to go back to being male. I'm just wishing that being female was easier.)