Capitalism. It convinces us that all that is good is bigger, faster, stronger, higher and the pursuit of such is a noble one, I'm not saying it ain't. We all happen to be stuck in a phase of existence that is fairly uneventful (no dinosaurs chasing us, no ice age, imperialism doesn't affect westerners). Consumerism gives us a good reason to keep plugging away: Ski trips, '15 plate Range Rovers, Mortgage in Richmond and one day the kids will be attending Haberdasher's blah blah.
So what does this have to do with you being so glum? Well, many years ago (i'm 35), I too was glum and my poor performance at uni (desmond at 25) left me feeling pretty useless. My 'strengths' or 'passions' just weren't compatible with peoples' version of what matters in the real world. I got a 2:2 because frankly my brain struggled to reconcile that uni was a productive endeavor for me, but I let the pressure of others keep me at Uni. I wanted to be an olympic athlete, so I trained my arse off for 15-20 hours a week, I wanted to be lead guitarist so I learned to shred and jam my own stuff around London and I wanted to own my own business, so I set up a catering company with my cousin, all between 18 and 25. I was compelled to do these things as they breathed life in to me. Not some arbitrary curricula.
I firmly believe most uni courses (pretty much just MBBS) have no use in the real world (unless of course research floats your boat). University in the 21st century is just an economic relic of New Labour, we have let people on platforms convince us that we need to jump through the university hoops. I'm not discouraging you from uni as it can help you off to a good start (not always), but 5 years after you leave uni it won't be a big deal, 10 years after no one will give a **** about your academics, they will only care about the value you provide. I have seen some incredible turnarounds of people I went to school with go on to lucrative careers after resitting GCSEs, doing GNVQs and barely finishing HNDs. The world of academics just wasn't a conducive environment to their strengths, but they were brilliant in so many other ways and once they came in to the real world and learned those other non academic things were of value, they grabbed the bull by the horns.
Modern education that we inherited from Prussia was routed in controlling and organising the growing youth population, eventually it was deemed a way of building vocations to deal with the sudden surge of skill gaps that the industrial revolution brought. It was never about 'you', so many people will not deal with it, their minds, personalities, strengths, passions, desires etc won't be nurtured by it so they will end up glum and depressed. To those folks I say, find stuff to fall in love with that will develop you, bring you to life: creative (sketching, music, photography etc), health (nutrition, blood chemistry/endocrinology),movement (running, weights, sports), logic (programming), entrepreneurship (online/offline), marketing (learn about how humans are compelled to buy) and psychology (learn about the human condition). Go and find like minded people, start a group (offline), connect in the real world in some way. Read and read books, not just online stuff, read books that will move the hell out of you. This will develop you and eventually you will start getting confidence in a world beyond the grades. It exists, it's so much richer than the world with grades
Btw, I failed, spectacularly at my goals of olympic glory, rock godness and being the next Branson. But, engaging in those pursuits educated me more than a dozen degrees and prepared me to provide value when I got to the real world.