Sometimes we don't know the impact of a thought until it comes out in speech. I may say something like, "I always mess this part up" and realize as it's coming out of my mouth that it is not a belief I want to perpetuate. I'm now observant enough of my speech that when I catch myself saying something I don't want to be true, I stop myself immediately by saying "nevermind".
It used to drive me crazy when people said "nevermind". I would pry and pry until they told me what they were thinking. It was like they were hiding something from me otherwise. Now anytime I hear that word, I feel so relieved. I want people to keep their negativity from me!
Saying "nevermind" works great in conversation, but how do you do the same thing in the conversations in your head? Another easy tool for stopping annoying or negative thoughts before they take over is to think "I'll think about that later." Whenever I find myself in a sticky puzzle that my mind is having trouble solving, I just say "oh well, I'll think about that later." And I let it go. Whatever the puzzle is always works itself out when I stop thinking about it, and I spend way less time mentally chasing my tail.