Hey, I get you.
I wrote the ELAT and attended two interviews with Trinity Hall Cambridge for English and they all went really well; I was well on track to getting an offer from Cambridge. Ended up messing my A-Levels (I was predicted 3 A*s and 1 A and I ended up with 2 As and 2 Bs. I've got no ******* clue what went wrong) and got rejected. I've got offers from Edinburgh, Manchester, and KCL but I only meet the offer requirements for Edinburgh. It's a global T10 school for English but you're right, nothing quite matches up to Oxbridge, especially when you've had your heart set on that.
We all come across bad days, despite priding ourselves on being smart. It's just unfortunate that our bad days had to make us do a complete 180. I don't really know what else to say, because I feel like a failure too. People do keep saying that eventually it'll feel better, but once again, you're right---so much potential was wasted. We can't really just get over the opportunity of our lifetimes slipping through our fingers in a few days.
I guess I replied even without words of advice 'cause I wanted you to feel less alone in this---I know I do.
You sound smart, one bad run can't and won't bring you down. It'll get better.
On that note, congrats on your other offers!