I think it's so important to have a little bit of self-reflection throughout your life, just because you might see things clearer and from a different perspective. With self-reflection you would come to the conclusion that most of the criticism you have towards other people, is actually a form of lack of self-confidence from your part. Of course, you would have to be some kind of superhuman being not to make any negative comments for the rest of your life, but having this thought in the back of your mind, will definitely help you finding out why you're saying what you're saying! It also helps you when other people having something negative to say about you! When this occurs, stop for a minute and let the following quotes help you shake those comments off!
"Criticism is an indirect form of self-boasting."
- Emmet Fox
"When we judge or criticize another person, it says nothing about that person; it merely says something about our own need to be critical."