I kind of agree with Mike here regarding Mahiro's own self-hatred. I do think the author intended this to be a dramatic moment where Mahiro willingly chose to continue being a girl, but considering the situation it isn't exactly a fair decision. If they chose to not take the drug and turn back, they'd be faced with having to explain themselves. Not just to Kaede and Momiji, but likely to the onsen staff. Taking the drug again was the path of least resistance. (I do want to voice that I'm kind of uncomfortable with the ongoing deception, particularly as it applies to Momiji -- hopefully the latter pieces it together and eventually confronts them. Crossing my fingers.)
I do believe Mahiro, though, when they say they were finding it comfortable. Whether that's from enjoying the feminine gender identity or just living in a more healthy way is open to interpretation. I'm not saying I think Mahiro is necessarily beginning to identify as a girl, just that they've enjoyed the role. It could very well be as Mike said and it being more about getting away from whoever they were, however possible. Or it could be a bit of all of the above, which is about where I'm leaning.
Or I could just be reading way too much into a comedy gender bender manga but ssssh I'm having fun