How I F***ed Up in Y Combinator

It's unfortunate that things didn't turn out better, but like you said, you're still young and you learned a lot. From your own selfish, subjective perspective, I'd say the YC experience was probably a good thing, no?

One thing about this stood out to me though:

On top of everything else, our mentors had just impressed upon us that our idea was stupid. So we pivoted.

I dunno, that just doesn't sit right with me. OK, maybe the idea really was stupid, I dunno. But it's the fact that you completely changed gears (that's more than "pivot" in my book) just because someone(s) told you the idea was stupid. I mean, even if those someone(s) included pg, sama, rtm, whoever... I couldn't see a complete reboot based on just their opinions.

Of course there may be context missing here, but I'm curious how much actual customer development effort you put in? Did you actually try to identify and interview the people you thought were in your target market? Was there any market research at all? If not, I can't help but wonder if the "pivot" here was a good idea or not.

Sorry if this sounds judgmental, and I freely acknowledge that there isn't enough context here to make many assumptions, nor am I in any position to criticize others. It just strikes me as odd to pivot so early and (seemingly) without sufficient data.

Looking back, we should have found a way to address the partners’ concerns. We should have found a way to use the code we’d written and the domain expertise we’d gained.

Looking at it from an outsider perspective, that sounds right to me. There's a lot to be said for acquired domain expertise and knowledge, IMO.