Always forgive, but never forget. Practice impulse control. Be wary of people who think they are your friend. It's easy to screw someone close enough to be your friend. Do not confuse knowledge with learning.
Stupid people serve an invaluable function in all of our lives.
By showing us what not to do, they make us better. You will be disappointed many times. But the times you are successful more than make up for the failures. Just make a decision. Every choice we make has consequences.