Ask HN: What is the biggest challenge you’ve faced when building your startup?

Building startup is difficult. I am now trying to build another, but having similar problems as always, which annoys me: finding product-market fit & early adopters. What are the biggest challenges you’ve faced? How did you tackle them? (Or did you manage to tackle them?)

I am asking it because if there is a pattern about those problems, and if they still exist, maybe we should do something to solve those common problems first.