I hate all the people that post in this thread without having been in similar situations and reply with bs such as 'OMG, youre horrible because theres 1000s people wanting to be in your position bla bla' or 'you can't quit, its oxford' when they cleary don't have any idea what theyre talking about (not that Oxofrd is now a secretive magical palce that onyl those inside it can fully understand its workings, far from it, but thas the opinion people on the outside have). No, Oxford/Cambridge is a university like many others.
The OP has no responsability whatsoever to those who failed to get in and she shoudlnt feel guility about them, this is real life, not pixie fairyland. The OP if she really thinks its not for her, she has every right to drop out without feeling pressure from people saying 'NOOOO, its Oxford, your life would be ruined' cause it wont.
That said, i also thought of dropping out, not as seriously as you probably, more like a lingering thought. If there are things you don't like, since Im guessing its the 1st year, remember you may never ever see them again- you only have to pass your prelims and burn all corresponding notes to it in a vengeful fire. Also, talk to the tutors, I was feeling quite gloomy myslef thinking I wasnt good enough until my tutor actually told me he thought I was good...which really changed my perspective.
Sicne what youre applying for is very different to your subject I think its a good idea you apply this cycle and then wait out, see if it gets better. That being said, do try to stay positive because if you think everyhting will stay the same i.e bad, chances are it will probably be that way.