Hi there Anon,
I'm really sorry to hear that you're feeling this way at the moment, but I think it's really important that you address your feelings to someone. It's terrible when parents and teachers are forcing you to take a certain path after leaving school, but they need to realise that it's your life to live, not theirs. Your parents aren't going to be the ones reading intensively every day, and your teachers aren't going to be the ones writing extensive essays every week. If your gut is telling you that this is the wrong decision, then the situation needs addressing.
Obviously, it's your decision to make, so I don't want to advise you either way, but I found myself in a very similar position last year (just at another university as opposed to Oxbridge), so I'll share some advice on how you can overcome this. It's important to assess the situation with a clear mind, so sometimes it's helpful to go back to the basics and write a simple pros-and-cons list of starting university this year against reapplying.
Whilst you're doing this, it's important to bear in mind that students attending Oxbridge do go out and have fun: you work hard but play hard. Yes, Oxford and Cambridge are more intensive than a good handful of other Russel Group universities, but this means that the students find even more brilliant ways to let off steam. Also note that every university requires more independence compared to A-Levels, however, at Oxbridge, you will also receive more personal tuition in the form of supervisions/tutorials. This means that a portion of your teaching will be tailored to you: such personal feedback on a weekly basis means that you certainly won't be doing your degree by yourself.
Consider the list you've made, and if your gut is still fighting against Oxbridge, then it might be a good idea to have a proper discussion with your parents. I would advise you to mention your mental health, as that is a critically important factor, but if you don't feel safe discussing that with them then speak to your counselor or a doctor instead. If you're suffering from your mental health to this extent, then you might not thrive at any university at this given time. If you feel this is the case, then a gap year to focus on your mental health and well-being could be the most productive thing to do. You could reapply in this instance, but it might be possible to defer your place for 2022 entry (although your university might disallow it). If you successfully deferred but then changed your mind, then you might be able to cancel your deferred place and reapply in the same cycle, although this isn't guaranteed.
Here's some more advice on deferrals:
https://www.thecompleteuniversityguide.co.uk/student-advice/applying-to-uni/deferred-entry-to-universityGood luck and I hope everything works out for you.