One thing that stood out to me is that you've applied 3 or 4 times as a solo founder, while acknowledging that YC prefers teams to solo founders. The reason they give for this preference is that "one-person startups are tough and you're more likely to succeed with a cofounder," which seems reasonable to me.
Have you tried in earnest to find others to apply with you? If not, why not? This may be the single most important thing you can do to improve your odds of being accepted.
If you have tried to find cofounders, but haven't convinced anyone, this may also be worth looking into. It's hard to say without knowing any details, but it's possible that YC may be passing for similar reasons as potential cofounders are.
Have you tried in earnest to find others to apply with you? If not, why not? This may be the single most important thing you can do to improve your odds of being accepted.
If you have tried to find cofounders, but haven't convinced anyone, this may also be worth looking into. It's hard to say without knowing any details, but it's possible that YC may be passing for similar reasons as potential cofounders are.