I precisely remember being in your shoes twice (waddling through a lot of teenage cringe I can dig up the links - 2018 (when I was in 11th grade; 12th graders were not allowed back then and I missed the cutoff by one mark), 2019 (when I was in 12th grade)). I did end up going to IMOTC though (that's a different story -- let's not digress), did my undergrad in math, and currently doing my math PhD now; so thought would throw in my two cents and what I would like to tell that version of myself.

I get what you're feeling - it sucks to not be part of that selected cohort of like-minded peers, who would obsess over doing cool math probletm for an entire frigging month. Or missing the chance of making the coveted IMO team. Or making math-sy friends. Or - let's be blunt here - getting the intellectual validation of feeling smart, that you get to be part of that "elite" cohort (which isn't really the case -- those who make it to IMOTC have an OK but not a great correlation with the "top X high schoolers in math from India"), especially if you don't have academic validations of this caliber before.

But trust me when I say it, a couple of years down the lane, it wouldn't matter really. If you want to pursue math-sy stuff (mainly talking about pure math/CS), the skillset that you built from Olympiad prep would pay dividends (and lot of people do much better when there's no exam pressure). And you would make math-sy friends in undergrad anyway (esp if you end up going to the typical top choices in India for maths -- and the entrance exams should now be a walk in the park). Many - or perhaps most - of my IMOTC friends are not doing math now anyway (most of them by choice, but for a nontrivial amount of them it's not by choice so it's not the silver bullet); and many people I know who are very good at math didn't really do that well in the Olympiads. Do not take not making to IMOTC as a blow to your self-esteem - there's a lot of noise of what happens on a particular day, and it usually evens out in the long term. And if you don't want to pursue math then it wouldn't matter even more.

Anyway do realize that not making to IMOTC is part of many failures that are inevitably going to happen in your life (not just in math/academics but in other aspects as well). Sounds cliche, but what you really can do is learn whatever you can from whatever went wrong, accept not everything is in your control, and keep trying for new opportunities.