Hardest Lesson As A Parent

Currently 27 weeks with my first. I've been doing some reflection of my time growing up and the influence I've had on other people's kids. What's the hardest lesson you've had to teach your kid? Was it hard on them, or was it hard on you? Why?

I think about my mother and how changing I was as a teenager with undiagnosed mental health problems. I also had a (mostly) absent father who instilled a complex of not being enough into me. My mom spent years teaching me that it doesn't matter what people who don't care about us think.