Coming to the realisation that I'm not as intelligent than I have been led to believe

We would have things like Maths Challenges, Physics Challenges, Maths Olympiads, Chemistry Olympiads etc. at school. These were tests of intelligence which went beyond the normal syllabus. I always ended up with terrible marks compared to everyone else. Other people would get silver and gold awards but I would end up with fails every single year (I think I got a bronze in one Maths Challenge and silver in one Physics Challenge but that was bad compared to what other people got). This shows all I was good at was learning from a textbook and learning from markschemes to do well in exams. I didn't have raw intelligence and the ability to solve problems or think outside the box.

OP, I've had a similar experience but from a different perspective.

A realisation has hit me over the past year or so. I am a perennial underachiever in nearly every aspect of my life - in fact, I don't know a single person who even comes close in wasted ability. Initially I wanted to blame a weak disposition, but now I realise it's more of a habit of laziness built around the fact that I was fortunate enough to be born with enough natural ability in enough areas to basically rest on my laurels from relatively early childhood. So now, finally, at the age of 21, in my final year of university, the reality of my situation has finally started sinking in as the people with the work ethic but without as much natural ability are starting to catch up to me / overtake me.

What I'm trying to say, OP, is that when talking about intelligence (or any other 'natural' ability - athleticism for instance) you have to include work ethic as part of the equation; if you don't work and underachieve then, regardless of your aptitude, your natural ability is irrelevant and, as other people improve, you'll become more and more average. You simply don't learn how to grit your teeth and work hard when the motivation isn't there to do so: you have no foundation to build your abilities on. I had it all so easy - to be honest I still do - but I'm not improving. I'm not living. I'm wallowing in self-pity thinking about what I could have accomplished by now with a better work ethic.

Talent never out-does hard work. It's a cliche, but it's true. So OP, you may not be as naturally intelligent as you thought you were, but if you have a stronger work ethic for it then you've got the one skill that matters most.

Don't sweat it mate. I'm not intelligent either, just a very hard worker. And I've achieved more than most 'intelligent' people have. You've done well so far, just use that work ethic, drive and motivation to your advantage.

There are loads of textbooks geared towards preparing for maths olympiads and challenges. A lot of the people who do well have studied for them. It's just a different syllabus and skill set from A level. You can't really answer some olympiad questions without a knowledge of things like Ceva's theorem in geometry, which isn't really covered in A level.

Also, just having practised more maths in general and having more experience of problem solving helps with maths challenges and olympiads.