There, I said it.
Startups aren't shangrilas devoid of problems you saw at a bigco. In fact, they amplify those problems in a small team. And you're going to be dysfunctional going forward dealing, accepting, and working around those problems.
Yourself.
Without HR, perfs, buses, or fridges full of Odawalla to soothe your pain. Maybe even without steady pay and health coverage. So yes, grip those cat meme pics tight, often it'll be your only distraction from being inside a fire.
Check your attitude first. Get that it's not you joining a startup it's equal parts a startup joining you. You decide where to cut the red tape--then deal with the political fallout of seeing the same 4 or 5 people every day. You decide when and where the crisis mode starts--then deal with the project failures. You decide the level of technical incompetence and smothering bureaucracy--then deal with the dilution.
Also get that YCombinator isn't just a vetting tool but rather a homework club. Passing out notes from upperclassmen and tips from product to launch to marketing to even... when to hire and not hire people that don't get it.
Be do go out and keep working. Find intelligent people working on problems you find interesting in a company that you can be proud of. Those are the real criteria, not labels, you'll need to worry about.
It'll take awhile though, and you're going to need a couple stumbles and wrong turns to figure out where that is.
Best.