Having a constant stream of reliable feedback from other people is the only way to ever find a place where we fit in. Not just that, but the feedback should be fair, as in, not overly dramatic and not overly critical. If you spent too much time without this kind of feedback, you can grow up into a maladjusted person that does inappropriate things, causing others to just ignore you, further intensifying the effect. In short, you're being ignored because people think you're mentally ill, which leads to you actually becoming mentally ill.
In my opinion, supported by thinking, reading and experiencing, the reason for this lack of feedback is the lack of parental attention at some point during life. Parents should have their heads screwed on straight and provide the essential encouragement needed to get the kid started out on a proper way through life. If that attention is missing, the kid starts thinking he's crazy, anti-social or just unworthy of attention.
This negative mindset becomes extremely powerful, leading to plenty of negative experiences that simply fortify it. In short, the kid isolates himself because the feeling of being ignored is too painful to experience again, which is a part of the reason for the attention vacuum. This kind of isolation is maddening, leading to the kid acting out in all sorts of ways.
At this stage, the problem is way beyond curing but not beyond remedying. The simplest way to remedy the attention vacuum is to simply be grateful every single day for everything that has happened. In my experience, feeling gratitude helps lower the bar for attention and makes the person realize that they are getting feedback, it's just that it's so subtle that it's very hard to detect right now.
The second remedy would be to react as little as possible to everything that's happening. If you feel people ignoring you, shut up. Don't talk to them about anything, don't discuss your problems, don't act up, don't try to convince anyone of anything. If you are being ignored, the ruling has already been made against you and the silence speaks louder than any words. A "hello" once every week or so is more than enough with such people or just pass them by and do your own thing. You shouldn't be trying to impress anyone, your task is simply to observe in the most inconspicuous way how others behave, gathering tidbits of feedback to patch up the critical neural pathways in your brain.
In 2019, I started going out to a bar, where I was accepted with open arms. I found that my "acting out" was actually quite appropriate in the bar, where people would accept it with mirth and play along rather than verbally cutting me down like my father did. I would basically get a round of applause when I'd walk into this bar, which did wonders for my self-esteem.
There are countless people out there that think you're mentally ill before you've said a single word based off of your hair, clothes, smell, laugh, teeth, eyes, fingernails, shoes and many other irrelevant details. That's their prerogative but you should not be wasting your energy on them. Instead, find such an environment where you can observe others, where those details don't matter as much, where you can release some of that pent up energy and just have fun.