Don’t pay, why the hell should you?
If you had full joint account set up then fine, but you don’t. Paying it is like accepting you living where you live is for your benefit, but if she was happy 6 months ago it’s a bit rich to ask now once she’s realised the cost.
I’ve been with my other half 9 years, lived together since July, pay 1/2 rent each & equal set amount into joint account each month to cover groceries, household stuff, tv licence, electric, council tax etc, but the rest is separate. Married friend does the same, works fine for all of us, as long as all the bills get paid, up to you what you do with the rest.
Only caveat I’d add is that the above only works if incomes/outgoings are similar.
Maybe the naivety of youth, and if/when I get married then fair enough, chuck it all in a pot (though it’ll not work out like that for me if I marry this one (family farm etc…..). I’m stubborn, and doggedly stand by my opinion, and just don’t do the giving in to mrs for an easy life thing, I’d rather have disagreements but know I’ve stood by my principles.