ps, I WAS in this situation a long time ago but I agree that ''moving out'' even if it were affordable might just create a whole new set of problems. Not having enough money, not having childcare.
It is better to learn, bit by bit, how to deal with your boundaries being infringed upon.
Upthread some posters having given some paragraph long suggestions about what to say. But I think it's not ''what you say'' that you need help with. It's learning to sit with that excruciatingly awkward feeling you get when you know your mum is upset with you because you have ignored her advice.
So rather than going in with a paragraph long manifesto ''please respect my boundaries etc i need to find my own way etc'' all your mum will hear is criticism and she'll think you're an ungrateful little madam and it will all end in hurt feelings and a bad atmosphere! what I recommend is that you hear one of her suggestions presented in terms of ''do this'' and quietly without offering up any reasoning or explanation or justification just do something else. Don't do it for the sake of it. Just wait until the moment comes, that moment where she is suggesting x but you want to do y. Do y. Don't prepare her for the fact that you have done y. Don't present ''a case'' for why you did Y. As that only perpetuates her belief that you need her approval to do y. And in you, it reinforces that uncomfortable feeling you have if she disapproves of your choice if it's not hers''.
So the next time she says ''do x'' and you ''do y'', do it with AS LITTLE EXPLANATION as possible. This is key. Do NOT explain it. If she asks for your reasoning or justification, as tempted as you will be to defend the rationale behind your decision making, do NOT give it. Just shrug and do Y.
You disentangle that enmeshment so that you can do what you want to do without feeling the need to explain yourself to her.
How she reacts to that is not a part of your process. Her martyred reaction may make you feel extremely uncomfortable but do not race to appease her. Just quitely bare it without discussion and it will get easier.
I can promise you that much. Use this as a learning curve.
Do you get an uncomfortable feeling if you ignore her advice?? Learn to ''sit with the discomfort'' without explaining yourself, as that is what sets you (both) on the path to disentanglement.
I never felt the need to explain every small decision to my brother so I used to say in my head '''if this were *brother, would I still be talking?'' and if the answer was no, I'd shut up.
Over time the dynamic has changed. My mother no longer expect me to do what she tells me to do, make the choices she is pushing on me.