Helloo PP,
I guess the first thing is to try and lessen your feelings of panic. It sounds as if the thought that you have too much revision to do for your Prelims is setting off a chain of escalating panic thoughts. Maybe something like -"That pile of lecture notes is too much for me to get through - I will not be able to answer the question in the exam - I will fail the exam - I will get chucked out of Oxford - my family will be devastated - I hate the subject I loved since I was a child - I have to put in for Clearing - I have failed at life- [continues to imagine the rest of life culminating in dying, impoverished, alone and unloved in a ditch] "
At least, this is what I felt revising for my Zoology Prelims many years ago. I also know this is what my daughter felt revising for Biology Prelims much more recently.
Some things that might help right now are:
1. Recognise that these are normal feelings. It is extremely unlikely that you are really behind everyone else in your year. The volume of material covered in the organisms course alone is impossible for a normal human to memorise - everyone who enters the Exam Schools building does so with massive gaps in their revision. Your problem is almost certainly that you are simply finding it a bit more difficult to live with those gaps than some others!
2. Try to find ways to interrupt the chain of negative thinking. For example, when you notice that thought, "There is too much to do...", practice replying to it with a thought like, "Yes, but that doesn't stop me from doing it anyway". If you think, "I am going to fail my Prelims", practice a thought like, "maybe I will, but that is no reason not to try" etc etc. Likewise when you feel nauseous from anxiety, notice the feeling and tell it that you are doing what you can, so go away, thanks.
3. Speak with your tutor(s) - email them now if you are at home - and ask to meet to explain your situation and ask for help to rationalise your revision. They might help you to narrow things down for you.
4. Speak with student welfare services. They are very used to helping students develop coping strategies.
5. Remember that you are at Oxford because you wanted to develop your love for Biology. You are not there to be bossed about by exam timetables and whatnot. Have a revision timetable, but include time to rest and relax. If anxiety feelings crowd into that recovery time, try to remind those feelings who is in charge here!
OK, I have written too much. Feel free to PM me if you like.