1) It's not talked about much but this is an AMA so let's do it. Having to change our management team as we scaled. You have so much loyalty to people who made you successful it is brutal to have to hire a different set of people as you change as a company. This is true for almost every founder CEO scaling a fast growing business. Here's Larry Ellison talking about it: https://www.youtube.com/watch?v=HzZOfoHzju4
2) Set your life up so that you can stick with building a startup for a very long period of time. If you're not ready to make that level of life commitment then I recommend you don't do a startup! One of the things I found when I was researching what it took to be successful was almost every great startup would go through a period in the first few years where rationally they should give up (eg Airbnb founders selling cereal). For whatever reason they didn't and went on to find massive success as through sheer persistence. We spent a year on voice recognition app Sonalight and it didn't work out. There was no question though that we would keep going with Amplitude. It doesn't matter where you start out as a founder, by sticking around enough you end up learning so much and getting more formidable over time. Eventually you outlast most other founders who quit and go on to find success.
3) I need to think more about this one. Most of the stuff I agree with, the challenge comes in understanding what it means in practice. What mantras are you curious about?