OP she's abusive and you need to find a way to build up your capacity to say no to her and cut her out of your life.
Presumably part of the reason you are worried about saying no to her is because you may need to borrow more money off her in the future?
I wouldn't send her any of the calculations people are putting up. In part, they don't sound that reliable. It sounds like, while you kept paying her back, each month the amount you borrowed increased, at least at first, as you didn't have enough coming in to cover expenses. But mainly because you shouldn't feel indebted to her for helping you out when she failed to give you the start a parent should. It's also likely she received child benefit for you (which is over £1k a year for an only child) while you were 17 and living away from her. Arguing with her over the amount is not the way to tackle this.
Is it still the case that you can't cover your expenses on your current income? Are you still borrowing from her? If you didn't have to pay her anything, could you get through the month?
If so, your reluctance to tell her no is somewhat sensible. Your first step in standing up to her is to get yourself in a position where you don't need to rely on her. Look at your budget, track every expense, cut where it's sensible to, and look at how you can increase your wages, take a second job if you can, find better paying work. Really focus your efforts on getting yourself financially secure. While you're at it, keep meticulous notes on what you borrow from your mum and and what you repay. This is important for financial management anyway, but also, I think it will help you a bit to see exactly what's going on. I suspect she is lying to you about what you owe. Seeing that in black and white and understanding how she's gaslighting you may help you break away.
If you don't still need to borrow from her, then you need to say no to the repayments. I think you should say something along the lines of "No, I've paid you back far more than is reasonable already. I'm not in debt in you.
But you could start with just, "no, I can't afford that much. I can manage £X if you really need it." Then I suggest if you pay her, each time you do, do so by bank transfer and text her saying "Have transferred you another £x, since you say you really need it." But don't ever say you are repaying a debt. If she ever texts, emails, or otherwise puts in writing that you owe her the thousands she says, you need to respond with something like "That doesn't sound right, mum." or the like. At some point you need to say, "By my calculations I only owed you £Y, which I've now repaid. I won't be paying more."
I would suggest, if you're going to go this route (and I wish you wouldn't, cutting her off will give you a better foundation for the rest of your life), that you estimate how much you used to borrow (not the £2 and £3 unless that happening 2+ times every week) from her and how much you have paid her back. Do not add any interest. Do not accept her figures.
You also need to see if you can access some counseling via the GP or a charity - you have had a really difficult entry into adulthood and don't have the resources (in terms of family) most people do. You could really benefit from some help understanding why you feel indebted to someone who has failed you so badly.
You have done really well so far - leaving home at 17 in todays society is really hard and you have survived and got yourself together. You're going to thrive. In 10 years (which will go far faster than you expect) you're going to look back on who you were and, I hope, feel proud of how you coped. But i think you'll also barely recognize the person you were. You will gain friends and find much better support. People who love you and don't treat you badly. You'll have a different way of looking at yourself and others. And you'll feel anger or sorrow (quite possibly both) for your mum.