I think the important question you should be asking OP is to yourself, "Why am I allowing myself to be treated this way?".
You have three children fathered by him, you run a household, you cook, you clean, you work part time, you more than contribute to the family that he and you have created, his regret concerning the third child is totally irrelevant, you didn't get pregnant alone.
I am a very strong believer in never asking, I believe a partner should offer, he knows you work part time, he knows how expensive food and essentials for children can be, he isn't stupid, he clearly knows you have next to nothing left each month for your self/for small extras. How do you deal with Christmas for example, does he contribute? Children's birthdays?
This actually makes me feel quite sad, to know that any woman (or even man) is being degraded and economically abused, you need to speak to him, it doesn't matter if he thinks he is "never wrong", you put your cards on the table and you tell him. No one is saying he should be left penniless but for fuck sake you should have a bit more each month, you need a bit more for a half decent standard of living, why shouldn't you be able to treat yourself from time to time? Or to stash an amount away for the future? Or for whatever reason.
He is a selfish prick and I hate all narrative on Mumsnet about "leave the bastard" etc but honestly you need to draw a line, he is a dick!