please do not let you mum define your own self worth based on her own successes. you are your own person and just because you are her child and the only other person in the household does not mean that your value to other people is the same stretched out expectations that your mother has. ignore the person saying that you're being too dramatic - that's not true and frankly speaking a little rude. I understand that it is hard when you have a parent who is putting you down when it comes to academics, but I would try to turn this into motivation to get yourself out of that situation: try to do the best you can in your three A levels - doing four does necessarily not make you more 'special', or 'smart' or 'study-oriented' or 'motivated' by most means - and if you want to apply for a lower entry uni as your last choice in case you really feel like it's not going to go well (and then you may have a much better feeling that you'll be out of this situation), then that is fine too. At the end of the day, once you're at uni you can control however much contact you have with her - and perhaps she may change her attitude once you're away and realise (one can hope). You are not a quitter because you're still doing your A levels - you just happen to be doing one less than you were before, and you're still taking the amount required for a standard uni offer. Quitting something normally means that you're now able to do something else - and in your case I'm hoping this means more breathing room and a lot less stress.
On a slightly lighter note, studying while wearing a face mask or sheet mask (and using the time it takes before you have to take it off to get one of your revision tasks done) is a fun way to study and also practice self worth and self care at the same time :,) Don't let your mum's words - however much she cares about you - diminish your own self worth.