Gender identity refers to how an individual experiences their "I". Nobody else but the individual him/ herself has access to a person's "I'. This "I" which we all have is the most fundamental experience of who "I" am. Absolutely nobody else can tell us how we should experience our sense of "I' because it is a subject and falls outside the domain of objective inquiry.
The almost universal criteria used historically to identify the biological sex of an individual at birth is the presence or lack of a penis. Presence of penis = boy, lack of penis =girl. The designation of the sex of an individual on their birth certificate reflects the categorisation of the individual's sex according to this criteria.If the biological sex of an individual cannot be changed then the results of using the same test to determine the sex of the individual at birth would remain unchanged no matter at what point the criteria was applied during an individual's life. If the same criteria is applied to a post op transgender woman her lack of a penis would mean that her biological sex would be determined to be female not male. Since the Gender Critical Movement's ideology declares that sex is binary - there is no spectrum- an individual without a penis and outer female genitals is by default designated female. A post op transgender woman is categorised as female therefore using the same criteria that categorised her as male at birth
This is such scrambled garbage! Firstly, sex is usually identified at birth. The biggest indicator of sex at birth is usually the genitals (only around 0.02% of babies’ sex is ambiguous at first glance and needs further testing to ascertain). However, it’s not just the genitals that make the sex. As an example, the guy years ago who had his penis bit/cut off was still a man afterwards, as is a man who has to have his penis removed due to cancer. Their male sex is encoded in every cell in their body. The fact they’ve lost part of their genitals doesn’t change that. Thus a transwoman who has their penis removed remains a male.
Back to your first point. A person’s sense of “I” is fine but sometimes reality is important. As an example, I in no way feel or look my age. I sometimes knock a few years off. However, if I’m filling in a form to get a new passport, I’m not entitled to put my ‘how I feel” age in, I need to, and do, put my actual age and date of birth. Likewise, I can’t ‘feel older’ than I am and claim my pension. Facts matter.
As to what you said about trans people living in fear, a lot of that fear comes from the very movement that’s supposed to be supporting them. This modern ‘trans’ movement has spread the idea that everyone hates them, and has played on the fears of vulnerable people, particularly the young, who are often dealing with additional trauma. Of course, there are always a few nasty people, but the vast majority of people don’t hate trans people or wish them harm.
The modern ‘trans’ movement has caused a lot of problems for trans people. The trans people I know in the U.K. hate it. They have no wish to impose on others, to live a life of perpetual victimhood, or to deny their sex. The movement speaking in their name is abhorrent to them. Trans people used to be able to live relatively worry-free lives, now these ridiculous demands and the huge proliferation of ‘trans’ youngsters who are nothing of the sort has ruined things for them.
People with sex dysphoria need support. The hangers-on who’ve politicised this and turned it into a Tumblr inspired pile of crap are the ones you should be blaming, not the vast majority of people, both trans and not, who understand that sex is real.