I wonder if the is-ought distinction can help us here. Which phrase better describes your feelings in the past: "desire to be female", or "perception of being female"? If it's the former, there's nothing to argue about and I will happily strike the word "delusion" out of my mind, replacing it with "desire", and everyone's happy. If it's the latter, what gives you confidence that your perception was correct in some sense, other than the perception itself?

people making conclusions about their own minds - and who can know your mind better than yourself?

Seductive, but false. Once my best friend and I disagreed about how I would react to a particular event. He was right. I would also say confidently that people online who claim to be vampires or stuff like that are, if they're sincere, making a mistake. Also, I know things about the psychology of suicidal thoughts that enable me to know important facts about the minds of a certain class of suicidal people better than they do. Also, there are people who deceive themselves about, e.g., being bisexual.

Moreover, if nobody can know my mind better than I, this is a big problem. It means psychology hasn't advanced enough. There's a doctor right now who knows my spine much better than I ever will, and this is how it should be. We need external specialized experts.

And most important - this is completely useless when you're trying to know your own mind. Some people regret transitioning and go back. When I worry about that, I can't shrug and say "Oh, but I can't be wrong!".