I missed out on an Oxbridge place by one grade. I was totally gutted - convinced I'd failed, my life was over. It was awful.
In the end I took a GAP year to figure it out, and it completely changed me. I am afraid I did the twattish thing (I didn't know it at the time) and went off to volunteer with poor kids in the developing world, and probably didn't make the world a much better place, but it massively opened my eyes. I'd been a fairly sheltered and privileged posh kid before that, with minimal interest in those less fortunate. After this, I was pretty sure I wanted to do something socially responsible with my life.
I went to a decent Russell Group uni the next year, and made the friends I am close twenty years later. I also kept volunteering while at uni with a number of projects, and have continued to both volunteer and work in the third sector, and then in academia and government work relating to an area I came across while volunteering.
I also changed track totally - my original degree was going to be law, but I changed my mind after my GAP year, did something different, and got a First, a Masters and a PhD in my chosen subject, which I'm pretty sure I'd never have done with law. I was just doing law because it was the right high status subject for really bright kids at my school.
Having that break made me really think about where I wanted to go, and chose something that mattered to me, instead of just following what I thought was the standard pattern for 'people like me'.
I also went to Oxbridge for post-grad which was quite nice for putting the old ghosts to rest. Not a failure - I just took the long way round!