I believe we don't have to tell you that insulting words are not good and I advise people to detest from using the word negative on ourself. While in the society we should be caution of what to say or watch over what come out of our mouth in the midst of people.
But there are word that should not be in our vocabulary while speaking or even In the midst of ourself. Those word will should evict are, i can't, am not, may be, and many more of this word we speak to ourselves. Normally we find those words coming out from our normal speaking while talking to people. But there is other side of this word which is literal meaning goes with impossible.
No man or human being on this earth should never wish him or herself bad with utterance you made with your mouth to your self. Let be careful with whatever that will comes out of our mouth.