I used to be like this and I'm glad that you've read some replies and you're trying to become less fixated on it. I luckily managed to get into Cambridge anyway, but there was a time in Year 12ish (before I applied) that I was so obsessed with getting in that it really affected my mental health and self worth. I thought that I'd be worth nothing if I didn't make it. Every time I didn't get a perfect score in a mock I would beat myself up so much... I felt like I had no future, that I hadn't done well enough, if I didn't get in. Like some people have said, I feel like it's the constant pressuring from schools/colleges and your teachers that sends you in the direction of that mentality. From the age of 13 my science teacher first suggested it and I got it in my mind that it was what I should be aiming for. I remember my French teacher in year 11 excitedly talked about it with me too. They made me feel like I should easily be able to do it? As if it was where I was meant for or something. Which is ridiculous, because applying and getting in is very, very difficult, and even extremely able people aren't selected sometimes, but they acted like it shouldn't be hard for me... so I thought if I didn't live up to that standard I'd somehow fallen short of the image people had of me and that really scared me. Because if I wasn't really as smart as they thought, that meant I was completely average, nothing special about me at all, a worthless person. I felt like all I had going for my was my academic talents.
In reality, there are some amazing people that get rejected from Oxbridge that go on to lead incredible lives after going to university elsewhere. There are still so many options that give you exciting opportunities and you're not worth any less if Oxbridge decide you're not for them and another place wants you instead. I think a lot of people have the mentality that it's the top or nothing, but Oxbridge are only ranked #1 in numbers and in peoples' brains, it doesn't mean they're best for you on paper. Other places can offer you an experience that's just as good, they can be your #1 university too. I managed to personally find courses in UCL and LSE that I'd love just as much after a good search, so after I got offers from them I was happily prepared for rejection from Cambridge if it was going to happen. It didn't happen, but it did happen to my friend, and she wasn't prepared like I was... she was still in that destructive mentality where it was 'the only place for her', and as a result we went to a party that week where she cried all over my shoulder all night and drank so much that she threw up all over the place and we considered calling an ambulance. So yeah, this isn't necessarily directed at you (I feel like you've already found other options) but if anyone else has that mentality, please, please do something about it, because it can end up pretty disastrous for your mental (and physical) health. This is a really long post but really I'm just using this as an opportunity to ramble about how badly that mentality affected me.