don't worry about it, you're definitely not stupid, it's just that Cambridge admissions is so so tough; there will my hundreds of other people with similar abilities who get rejected, and it is in no way a comment on your intelligence. I'm sure many other universities will love to take you, by the looks of it you seem like a really high level student. It may not seem like it now, but the rejection may be a blessing in disguise; if you weren't a good fit for Cambridge then i think you'll be happier elsewhere. don't worry about it, and try not to doubt yourself (as cheesy as that sounds)!!