I'm feeling like I'm in quite a similar situation; I've sat 8/9 of my exams and honestly whilst I think some of them went well, I also think some of them went really badly. I know it's hard to stay positive but I know that whatever happens on results day, you'll regret spending your summer holidays stressing over the grades that have ultimately already been decided for you the second that you closed the paper. I'm at peace now with my exam performances, partially after speaking with others and getting the validation that my answers seem right (although I do three essay subjects so it can be hard to know what that correlates to mark-wise), and also because I know that my subjective opinion of 'doing badly' isn't always the same objectively. I'm sure you can think back to a time where you thought a test went badly and you ended up coming away with an A or A* - the same can (and likely will) happen for the real exams because naturally your exam anxiety is heightened due to the stakes that these exams hold. At the end of the day you have a Cambridge offer which means that they know that you have the academic ability to study there, and whilst it might feel like it means nothing if you don't meet the offer, that's more than most people can say. This does also mean though that when it comes to results day, if you haven't got the grades you need, your college/another college are more inclined to accept you than many other unis would be because in essence they've already judged you at interview and have decided that they want you in their cohort. I'm hoping to study geography and from the UCAS acceptance grade tool, 55% of offer holders were accepted with A*A*C, and even more with A*AB/A*A*B, so hope really isn't lost yet!
Hopefully the rest of your exams are able to go better, and see you in October fingers crossed x