"You have no idea how long something you say can stay in someone's mind."
It is human nature to remember hurtful comments more than the positive ones. Remember that when your urge is to say something unkind. You could do one hundred nice things for someone, and it will be that one hurtful thing that will leave the biggest impression.
It takes strength of character to be kind, and it takes mental strength to change your own inner dialogue to a more positive one. This is the place to being since how you feel about yourself is often reflected in how you treat others.
Don't mistake kindness for weakness. Be strong. Be kind.