At 21 you should be living your best life - not worrying about providing childcare when you don't even have any children, paying for treats and trips for them, all on top of giving your mother 50% of your wage and, keeping house. I feel very sad for you. You're clearly doing your best having worked, and now chosen a career and studying & training hard to get there - this is far from easy with abuse and uncertainty at home. Been told to move out on an almost daily basis (whilst happily taking £400 a month from you!), is a monumental stress to be under when you're trying to better your circumstances. It's also just not fair. What a horrible way to make your daughter feel, 21 or not - not much of a mother I have to say.
A lot of people your age study and live at home - well into Uni, all the whilst staying at home - people commenting here forget that it seems. Not having support at home, even into your early 20's if needed, has a massive social-economic impact for the future - and, let's be honest here, compassionately, on people as individuals as well.
Telling her to work 7 days a week is ridiculous. OP will need adequate rest in between working, studying, keeping house, looking after children, and battling with her mothers (very obviously very poor, and very up & down) mental health almost every day.
I was in a similar situation to you OP, once upon a time. I was a bit younger but suffered through uncannily similar abuse that you have from your mother, and the financial abuse (everything was taken - and I was accused of lying about what I was earning). Even any birthday money I got was taken. I distinctly remember opening a birthday card from my father with £100 inside. The first bit of money he'd given me for a birthday or otherwise, in years. My mother 'asked' (told) me if she could have it as she was skint, I said 'No - this is for me. This isn't fair, I opened the card less than 1 minute ago and you're already trying to take my birthday money from me' and the treatment I got for the next week made me forfeit my money just for a stable and non threatening atmosphere. I daren't have gone shopping with it - I think I would have been locked out of the house.
I really empathise with you, and really understand, OP. Other posters commenting how you're an adult - clearly didn't share your upbringing, so wouldn't understand how abusive mothers like this are very manipulative and as a result, very successful at keeping their children 'young', not so knowledgeable about their rights (it's absolutely obviously in their best interests to make you believe you have none, other than the situation you are in with them) and erode your self esteem so well that you truly get to a point of believing you can't make it on your own.
She is wrong.
I won't advise anything else as you've received plenty - but I wanted to provide some genuinely empathy here, from someone who went through extremely similar. You will be OK. This treatment from her will leave a big mark on you though, and it's important to seek help for that.