gender is partly a social construct

gender isn't a social construct. gender is innate. Gender roles and presentation are a social construct, what one gender does and is associated with is absolutely socially constructed. However, gender itself, as in "I am female" isn't, it's almost the same feeling as proprioception, unless it's off, unless it's broken, you do not think about this actively. so what is Proprioception, it's knowing what parts of your body are where and what orientation they are in relation to the rest of your body without seeing them. Gender itself is much the same, and when everything lines up, you don't even think about it, it's just done. However with transgender people, our "gender proprioception" is off, it's not working with the map we've been given, so we stumble and fall. You can learn how to act male, or female, but unless you are that, it won't feel right, much the same as when you lose your proprioception and can't move your hands without being able to see them. This is easily fixed with transition.

However, onto detransitioning, as I understand it, since I can't imagine living my life as a man anymore, is mostly down to acceptance in that person's life. You often hear "regrets" as "Well, I got so many looks of dissaproval" "I lost so many jobs thanks to being trans". Rarely is it ever "I'm actually cis." or "I'm just depressed". This actual minority does exist, but of the percentage of people whom do detransition, because of "i'm just unhappy regardless" or "I am cis" is basically inconsequential. Most trans people are indeed quite happy after transition, especially when accepted, acceptance is a gigantic key feature to any LBGT minority's happiness. The more accepted the happier we are, the more we're treated as just another person, the happier we are. However I'm gonna stab, and say the documentary that you watched was probably Russian in nature. Generally, anything from Russia and referring to LBGT, it's only going to be hit pieces, it's only lies and bullshit. It's sad but the Russian government made that specifically because they want to delegitimize the idea that you can even be homosexual, let alone being transgender. I'd rather not have to say that about the Russian government but it's the truth.