Momentum is everything in a startup. If you have momentum, you can survive most other problems. If you do not have momentum, nothing except getting momentum will solve your problems. Founders internalize this during YC; many seem to forget in the few years after YC. Burnout seems to almost always affect founders whose startups are not doing well, and then becomes a downward spiral. In fact, one of my top few startup commandments is “never let the company lose momentum”.
Ref.: https://blog.samaltman.com/the-post-yc-slump