to think no one can define living as a woman

From gov.uk:

Evidence of living in your gender for the last 2 years
You’ll need copies of evidence to show that you’ve been living in your affirmed gender for the last 2 years.
Each piece of evidence should contain at least one of the following that matches up with your affirmed gender:

• a name
• a title, like ‘Mr’ or ‘Miss’
• a gender marker, like ‘male’ or ‘female’

• evidence from different points over the 2 years, with roughly 1 piece of evidence for every 3 months
• at least one piece of evidence from the last 2 or 3 months
• evidence from a variety of different sources

• your driving licence
• your passport
• identity cards, like workplace IDs, student IDs, railcards, health insurance cards, library cards or supermarket loyalty cards
• letters from solicitors, accountants, doctors, dentists or employers
• bank statements
• payslips, P60s and P45s
• benefit letters
• tax letters and documents, including council tax
• credit reports
• confirmation letters that you’re on the electoral register
• student loan statements
• utility bills, such as internet bills
• rental agreements
• academic certificates and documents

Just like no one can define woman in a way that means some male people are women. 'Trans women are women' only works if you un-couple the word women from its actual definition and after years of asking no one has yet supplied and alternative meaning that doesn't rely on the idea that woman means people who say they are women.

The only way to live as a woman is to be born female and grow up.
Anything else is reductive, outdated stereotypes.

YABU for starting yet another thread on mumsnet about this.

I agree with your point, but this has been done to death on here.

I can't even define what it is to live as a woman as a biological woman.

I know what it is to live as me. I am a woman. I do not represent all women and could not begin to define what a woman is in any meaningful way beyond the biological.