I have the feeling you are struggling to accept that you failed on that process because of your expectations and are looking for validation that you aren't a terrible developer, given that you even shared the age you started coding as if you need to show us social proof. So you've written that post to help deal emotionally with that problem. So no, you aren't a terrible developer, does it help you feel better?

First of all, recruitment and processes are random, have you interviewed enough? Most people old enough have interviewed for 100+ companies, and with experience you learn that doing well doesn't mean you'll get a job.

We're also in some sort of tech recession for non-AI jobs. Many companies keep their job positions posted and interview people but don't hire them, as they'd like to keep recruiters and the team busy. I know this because I'm in a leadership position and talk with many colleagues who are also in other companies leadership position. (not that my company is doing that, though)

There's another reason, you might think you have ace'd it, but the interviewer might have different expectations, also, just by solving the solution or problem it doesn't mean you are fit for the company, sometimes we think we aced it, but they expected a different solution that you didn't know. Dunning kruger effect?

You've mentioned yourself that you've done 75% in a few, this might be enough for they to chose a different candidate.

Also, if your CV is weak, they might have changed their minds, not every company wants to train people, management could have pushed a different direction to the recruiting process or just the budget could have been closed.

To sum up, just move on and don't waste time on this, maybe focus on studying more? Nothing in life is guarantee'd, you might study years and never get that HFT job, or you might get it. That's life. Thinking about it won't get you the job, what you can do is practice more until you make it.