I've been building startups for a long time, and have applied to YC several times, mainly with the same idea (although I've always listed 4-5 other ideas I'm interested in as well). I have always gotten rejected, and while that was disappointing, I 100% understood that getting into YC was the exception rather than the rule, as YC had a smaller acceptance percentage than Harvard.
After a few batches of these rejections, I decided to take a break from applying and instead build up my startup experience more by working at growth-stage startups, and just in general started focusing on my career as well.
Fast forward a few years, and about a year ago YC reaches out to me (via automated email) saying they had identified me as being in the top X% on the Cofounder matching platform, and encouraged me to apply to their next batch cycle.
I was reluctant to do so -- I hadn't been thinking too deeply about any new startup ideas in the past few months (so I'd most likely have to apply with a previous startup I had built), but after encouragement from friends and family I decided to take a chance anyways and apply once again.
I went all out on the application -- I reached out to some YC friends and mentors to get their recommendations, and had them help me edit and perfect my application as well. I even traveled for a month to San Francisco, just so I could absorb more of the ecosystem, network with as many entrepreneurs as possible, and help give back to the community if I could.
I submitted my application, waited for the fateful "interview email" day, and when that day came -- I got the standard "...we're sorry, we're not going to move forward with your application".
I'll admit -- getting that email stung. I know looking back there were a lot of things I could have done better (e.g. not applying with the same idea as previous applications, among other things), and just like when I applied years ago, I knew that the chances of getting to the interview stage were slim to none.
But yet somehow, that rejection hurt more than any of the others.