What’s one investing principle you wish you understood earlier?

I have been reading a lot about investing lately and notice that most people's advice boils down to a few key principles, but everyone seems to have one specific lesson that really changed how they approach the market.

For those who have been investing for a while: what's one concept, strategy, or mindset shift that you wish you had understood earlier in your journey? Could be about risk, time horizon, valuation, or even just behavior.

Hoping to learn from the experience in this community.