@Minglingpringle is right.
We all have a tendency to compare ourselves with the more popular and better-off - but there will always be someone in our social circle who has more friends and is more successful.
However - we rarely compare ourselves to the lonelier or worse-off, and think how fortunate we are.
We have to train ourselves to do this - to get a better sense of how our lives actually are.
It sounds as if you are accepting your daughter's version of reality, but it really, really isn't the whole picture!
She has won life's lottery - I don't just mean 'getting into Oxford' (blah blah blah), but having a first-class education, growing up in one of the world's best cities, being loved and cared for by her parents, having an excellent brain, having strong friendships at school and good friendships now... and she's now moving into the long summer break.
My young person, just a year or two older, has had life-changing health news recently and is grieving. It is hard, because it can't be changed, only come to terms with.
Sometimes life sucks, but other times we may have more than our fair share of good things (though funnily enough, we rarely think that that is unfair or unearned!)
It's so hard to help them, because they are adults now and some things just have to be learned, and sometimes things that aren't fixable just have to be borne.
But - what has worked for me, in my own life, is remembering that we have a 'duty of hope' to ourselves. If we forget that, we may fail to squeeze the most out of all the great things we do have.
She needs to trust that she will be fine. It may be hard, but she can do this, and she will be fine.