If a gender diverse person tells me that their gender changed, then their gender changed.

The problem is that you're assuming the past gender of trans women categorically.

But to say that the only way for trans people to be is the way you perceive them to be

I am quite explicitly not saying this.

You said:

She wasn’t born a man. Trans women don’t “become” women. They stop hiding the fact they are

I'm trying to tell you that this claim is not just wrong, but it's a harmful over-generalization. I was not a girl for the first ~12-16 years of my life, and that's why I'm telling you our identities (past and present) are ours to determine, not yours. I was a boy, I did become a girl. I was comfortably cis for a long time, but things changed as I grew up.

Reading this stuff today only serves to drag those anxieties back up, because if we assume your claim is true then I either wasn't a boy back then, or I'm not a trans woman now. I have to sit here and think about how you're wrong, and why trans people don't have to have always been trans in order to transition and be valid.

That's what motivated me to respond to you. Whether or not you're willing to grapple with the idea that you may be hurting people is your prerogative, I'm just telling you those claims have done harm to me in the past and may be harmful to others now, specifically young cis people.