How to deal with the stress of knowing you're not as smart as you thought you were?

I can't get over the fact that I'm not as smart as I thought I was. Throughout Year 10, everyone thought I was really smart because of some random lucky moments in class where I could recall the information. My classmates called me a 'genius' even though I was not a genius at all; for competitions like the Ukmt I would get bronzes while my classmates get Gold and move onto the next level. But for some reason my classmates continued to call me a genius. Even my parents and teachers thought I was a genius because I once got > 90% on one of my biology end of year mocks, the highest in our class, but the reason why I did was because I already knew all the answers, because I had done several past papers online and all the questions that appeared, I had already encountered in past papers. I feel ashamed and I feel like an imposter because I'm not as smart as everyone expected me to be. I have an average IQ, barely any extracurriculars etc and no real achievements.
Fast forward to today, every time I try to complete some extracurriculars, I can't help but feel paralysed when I see everything people my age, and below my age, have accomplished. Applying for programmes, I don't get the spot I want and I try to justify it, saying that maybe it's because I'm in Year 11 so I'm too young to have the spot, or the school I went to was not as good, or that my parents weren't that smart. But then I see Year 11s and Year 10s, in the same position as me, getting those same spots with much much more accomplishments than I have ever achieved (winning Olympiads in Year 7, playing instruments for 10+ years, already doing the programme before at a younger age). Every time I feel I have any advantage over academics, I realise that students much younger than me have already accomplished or are accomplishing it. I feel so guilty for being so incompetent. This guilt just further develops when I dream about going to prestigious schools like Harvard, getting perfect scores on the ucat, impressing everyone with my achievements, becoming incredibly rich etc, and then am thrown back into the reality that these dreams are just futile. I'm never going to accomplish these dreams because I'm NOT a genius, im an average person, and there is only so much I can do.
I'm crying as I write this because I just don't know how to deal with the feeling of not being what I thought I'd be. I don't think I can even turn back now; I practically based my entire identity on academics and I have a horrible social life and friends that dont really like me because I always let academics get in the way. I don't want to live the rest of my life feeling this guilt weighing me down. Throughout Year 10 I lived with the thought that I was the best and it turns out I'm just a fool. How do you deal with this realisation. I wanted to finally impress my family and friends and teachers. Also, it is necessary that I complete extracurriculars for my university application but how do I cope with the fact that the people on these extracurriculars are maths geniuses and incredibly academic students while I'm an average, ordinary person? The competition is overwhelming. There are some students I know starting extracurriculars as early as Year 8, while I can't even find any extracurriculars to complete in Year 11. What do I do. I'm really lost and I've been stuck on this feeling of guilt for the past 8 months. How do I stop overestimating my abilities and stop hoping for unrealistic goals? How do I control my stress so that I can actually do something productive instead of having a breakdown for hours? It's so stressful when you feel like you've finally stumbled upon something unique that you think no other applicant will have known about, only to find out that hundreds of applicants do know.
How do I deal with all the overwhelming competition? How do I accept that I cannot be the best and that thinking of myself as a genius is a lie? I'm really trying to forget all of these events mentioned but I just can't.