Startup #1: too hard to believe it could be big. Widely loved consumer website we tried to fund with angel/VC money, and it was too implausible it could ever be a $1b+ company. We needed a non-VC strategy to fund that business but didn't realize that - ran out of gas.
Startup #2: found PMF, business-model fit, high-growth, and had an amazing team. Blown up by a nightmare original founder (I was part of re-founding an existing business) and poor corporate governance. The original founder blew up a Series A and re-cap after the team new team made the company actually work.
What I learned from both is how finicky startups are and why conventional wisdom exists. The more you try to re-invent things that aren't core to the business or fit a square peg into a round hole, the more chances to fail you create.
Don't try to get 1000 things right. Try to get 1 thing right and rely on the wisdom of others to get the other things "close enough" to right to attract talent, capital, time, and attention to the 1 thing you did get right.