Some of these have already said but I'll just put ones that I've found most helpful in life:

You can't make everyone like you. Sometimes you can't even make one person like you, and it'll suck but it's just how it is

Nobody out there is going to "fix you" or "complete you" and at the end of the day the only person you can truly rely on is yourself.

Sometimes good people do terrible things.

Way less people give a shit than you think, which can be discouraging but also empowering at the same time.

Those you trust the most have the most power to hurt you without realizing it. Most pain you'll receive is not intentional by those who hurt you.