I'm cribbing shamelessly from Mighty Girl. Here are my favorite pieces of advice.
There are two sides to every story. Make sure you've heard both before you make your decision.
When I was a wee thing my grandmother filled out one of those Grandmother Remembers books. There was a page in there for my grandfather to write a few words, and this is what he wrote (also that he loved me and would always be proud of me). It is bar none the best piece of advice I have ever received in my life. I don't always follow it, but I should.
If this is the worst thing that ever happens to you, you're damn lucky.
Not so much advice, but a maxim for living from my grandmother. This has gotten me through some ugly situations.
Life is hard.
I remember reading this during one particularly hellacious winter, and instantly my whole body just relaxed. Yes, I thought. Life is hard. This is okay, this is normal. Life is funny and wild and unpredictable and full of joy, but life is also hard. The universe doesn't owe you an easy path. The earlier you learn this the more prepared you are to deal with that reality.