What's one startup lesson you had to learn the hard way?

I'm a marketing tech builder. I've sold a startup, angel invested in a few, and am now building another.

Some things I believed 10 years ago turned out to be completely wrong. Others took me years (and quite a bit of money) to understand.

A few lessons that stuck with me:

• Angel investing is probably the fastest way I've found to lose money.

• Your first business partner matters more than your first customer.

• The bottleneck is almost always product-market fit, not marketing.

• Hiring someone to "run the business for you" is much harder than it sounds.

• Build where you have an unfair advantage. Passion alone won't save you.

• Knowing when to back down is just as important as knowing when to push harder.

• I've never seen anyone succeed long term without working incredibly hard, regardless of talent.

I'm curious about two things:

What's a lesson you learned the hard way that nobody warned you about? And which of mine do you disagree with?

Especially interested in hearing from people building their second or third company.