Don't act like you knew someone if you didn't know their past.
Don't tell someone who they should be.
Don't tell someone what they should do.
Don't tell someone that you care if you're not going to be.
Don't act like you're the most miserable person in life.
Don't act like you're wise and always right.
Don't act like you know what everyone needs.
Don't act like you're a true friend of someone you didn't really understand.
Don't search for a reason if it's definitely your fault.
Don't search for mistakes that other people made.
Don't lie to yourself about things you shouldn't do.
Don't lie to a person if you've made them believe in you.
Don't make a promise you can't even keep.
Don't make someone cry if you can't stop it.

Don't. Ever. Repeat. The. Same. Mistake. Twice.